diff --git a/reproman/distributions/base.py b/reproman/distributions/base.py
index 6f56083bb..c4bfa6577 100644
--- a/reproman/distributions/base.py
+++ b/reproman/distributions/base.py
@@ -46,8 +46,6 @@ class SpecObject(object):
     _diff_cmp_fields = tuple()
     # Fields of the primary interest when showing diff
     _diff_fields = tuple()
-    # Fields used in determination of comparison (satisfied_by and identical_to)
-    _comparison_fields = tuple()
 
     @property
     def _diff_cmp_id(self):
@@ -60,12 +58,19 @@ def _diff_cmp_id(self):
 
     @property
     def _cmp_id(self):
-        if not self._comparison_fields:
+        if not self._diff_cmp_fields:
             # Might need to be gone or some custom exception
             raise RuntimeError(
-                "Cannot establish identity of %r since _comaprison_fields "
+                "Cannot establish identity of %r since _diff_cmp_fields "
                 "are not defined" % self)
-        return tuple(getattr(self, a) for a in self._comparison_fields)
+        if not self._diff_fields:
+            # Might need to be gone or some custom exception
+            raise RuntimeError(
+                "Cannot establish identity of %r since _diff_fields "
+                "are not defined" % self)
+        vals = [ getattr(self, a) for a in self._diff_cmp_fields ]
+        vals.extend([ getattr(self, a) for a in self._diff_fields ])
+        return tuple(vals)
 
     @property
     def _diff_vals(self):
@@ -94,8 +99,8 @@ def diff_subidentity_string(self):
 
     @property
     def identity_string(self):
-        """like diff_identity_string, but for _comparison_fields (used in 
-        satisfied_by comparisons)
+        """like diff_identity_string, but for both _diff_cmp_fields and 
+        _diff_fields (used in satisfied_by comparisons)
         """
         return " ".join(str(el) for el in self._cmp_id if el is not None)
 
@@ -137,15 +142,15 @@ def _satisfied_by(self, other):
         spec object.
 
         We require that the values of the attributes given by 
-        _comparison_fields are the same.  A specobject with a value of None 
-        for one of these attributes is less specific than one with 
-        a specific value; the former cannot satisfy the latter, 
-        but the latter can satisfy the former.
+        _diff_cmp_fields and _diff_fields are the same.  A specobject 
+        with a value of None for one of these attributes is less specific 
+        than one with a specific value; the former cannot satisfy the 
+        latter, but the latter can satisfy the former.
 
         TODO: Ensure we don't encounter the case where self is completely 
         unspecified (all values are None), in which case satisfied_by() 
         returns True by default.  Perhaps this is done by making 
-        sure that at least one of the _comparison_fields cannot be None.
+        sure that at least one of the _diff_cmp_fields cannot be None.
 
         TODO: derive _collection_type directly from _collection.  This isn't 
         possible at the moment because DebianDistribution.packages is 
@@ -162,9 +167,7 @@ def _satisfied_by(self, other):
             raise TypeError('don''t know how to determine if a %s is satisfied by a %s' % (self.__class__, other_collection_type))
         if not isinstance(other, self.__class__):
             raise TypeError('incompatible specobject types')
-        for attr_name in self._comparison_fields:
-            self_value = getattr(self, attr_name)
-            other_value = getattr(other, attr_name)
+        for (self_value, other_value) in zip(self._cmp_id, other._cmp_id):
             if self_value is None:
                 continue
             if self_value != other_value:
@@ -176,14 +179,12 @@ def _identical_to(self, other):
         """Determine if the other object is identical to the spec object.
 
         We require that the objects are of the same type and that the 
-        values of the attributes given by _comparison_fields are the same.
+        values of the attributes given by _diff_cmp_fields and _diff_fields 
+        (dereferenced in _cmp_id) are the same.
         """
         if not isinstance(other, self.__class__):
             return False
-        for attr_name in self._comparison_fields:
-            if getattr(self, attr_name) != getattr(other, attr_name):
-                return False
-        return True
+        return all(sv==ov for sv, ov in zip(self._cmp_id, other._cmp_id))
 
 
 def _register_with_representer(cls):
@@ -423,3 +424,101 @@ def _create_package(self, **package_fields):
         provided by _get_packagefields_for_files
         """
         return
+
+
+class SpecDiff:
+
+    """Difference object for SpecObjects.
+
+    Instantiate with SpecDiff(a, b).  a and b must be of the same type 
+    or TypeError is raised.  Either (but not both) may be None.
+
+    Attributes:
+
+        a, b: The two objects being compared.
+
+        If _diff_cmp_fields is defined for the SpecObjects:
+
+            diff_cmp_id: The _diff_cmp_id of the two objects.  If 
+                        _diff_cmp_id are different for the two objects, 
+                        they cannot be compared and ValueError is raised.
+
+            diff_vals_a, diff_vals_b: _diff_vals for a and b, respectively, 
+                                    or None if a or b is None.
+
+        For collection SpecObjects (e.g. DebianDistribution, containing 
+        DEBPackages; these have _collection_attribute defined), we also 
+        have:
+
+            collection: a list of SpecDiff objects for the contained 
+                        SpecObjects.
+
+            a_only: SpecDiff objects from collection that only appear in the 
+                    first passed SpecObject
+
+            b_only: SpecDiff objects from collection that only appear in the 
+                    second passed SpecObject
+
+            a_and_b: SpecDiff objects in collection that appear in both 
+                     passed SpecObjects
+
+    If a and b are lists, they are treated as files specifications, and 
+    self.collection is a list of (fname_a, fname_b) tuples.
+    TODO: give files and file collections their own specobjects
+    """
+
+    def __init__(self, a, b):
+        if not isinstance(a, (SpecObject, list, type(None))) \
+            or not isinstance(b, (SpecObject, list, type(None))):
+            raise TypeError('objects must be SpecObjects or None')
+        if a is None and b is None:
+            raise TypeError('objects cannot both be None')
+        if a and b and type(a) != type(b):
+            raise TypeError('objects must be of the same type')
+        self.cls = type(a) if a is not None else type(b)
+        self.a = a
+        self.b = b
+        if self.cls == list:
+            a_collection = set(a)
+            b_collection = set(b)
+            self.collection = []
+            for fname in set(a_collection).union(b_collection):
+                if fname not in a_collection:
+                    self.collection.append((None, fname))
+                elif fname not in b_collection:
+                    self.collection.append((fname, None))
+                else:
+                    self.collection.append((fname, fname))
+        else:
+            if self.cls._diff_cmp_fields:
+                if a and b and a._diff_cmp_id != b._diff_cmp_id:
+                    raise ValueError('objects\' _diff_cmp_id differ')
+                self.diff_vals_a = a._diff_vals if a else None
+                self.diff_vals_b = b._diff_vals if b else None
+                self.diff_cmp_id = a._diff_cmp_id if a else b._diff_cmp_id
+            if hasattr(self.cls, '_collection_attribute'):
+                self.collection = []
+                a_collection = { c._diff_cmp_id: c for c in a.collection } if a else {}
+                b_collection = { c._diff_cmp_id: c for c in b.collection } if b else {}
+                all_cmp_ids = set(a_collection).union(b_collection)
+                for cmp_id in all_cmp_ids:
+                    ac = a_collection[cmp_id] if cmp_id in a_collection else None
+                    bc = b_collection[cmp_id] if cmp_id in b_collection else None
+                    self.collection.append(SpecDiff(ac, bc))
+        if hasattr(self, 'collection'):
+            self.a_only = []
+            self.b_only = []
+            self.a_and_b = []
+            for pkg_diff in self.collection:
+                if isinstance(pkg_diff, tuple):
+                    (a, b) = pkg_diff
+                else:
+                    a = pkg_diff.a
+                    b = pkg_diff.b
+                if not a:
+                    self.b_only.append(pkg_diff)
+                elif not b:
+                    self.a_only.append(pkg_diff)
+                else:
+                    self.a_and_b.append(pkg_diff)
+        return
diff --git a/reproman/distributions/conda.py b/reproman/distributions/conda.py
index ef13eb093..8a881ae75 100644
--- a/reproman/distributions/conda.py
+++ b/reproman/distributions/conda.py
@@ -136,6 +136,7 @@ class CondaDistribution(Distribution):
     environments = TypedList(CondaEnvironment)
 
     _cmp_field = ('path',)
+    _collection_attribute = 'packages'
 
     def initiate(self, environment):
         """
diff --git a/reproman/distributions/debian.py b/reproman/distributions/debian.py
index 6c9ad9b35..3d33d4a5b 100644
--- a/reproman/distributions/debian.py
+++ b/reproman/distributions/debian.py
@@ -89,7 +89,6 @@ class DEBPackage(Package):
 
     _diff_cmp_fields = ('name', 'architecture')
     _diff_fields = ('version',)
-    _comparison_fields = ('name', 'architecture', 'version')
 
 _register_with_representer(DEBPackage)
 
diff --git a/reproman/distributions/redhat.py b/reproman/distributions/redhat.py
index 8f7baf0c4..87a48fefe 100644
--- a/reproman/distributions/redhat.py
+++ b/reproman/distributions/redhat.py
@@ -65,7 +65,8 @@ class RPMPackage(Package):
     vendor = attrib()
     url = attrib()
     files = attrib(default=attr.Factory(list), hash=False)
-    _comparison_fields = ('name', 'version', 'architecture')
+    _diff_cmp_fields = ('name', 'architecture')
+    _diff_fields = ('version',)
 
 
 _register_with_representer(RPMPackage)
diff --git a/reproman/distributions/vcs.py b/reproman/distributions/vcs.py
index b118fd48c..200f8845e 100644
--- a/reproman/distributions/vcs.py
+++ b/reproman/distributions/vcs.py
@@ -125,6 +125,8 @@ class GitDistribution(VCSDistribution):
     _cmd = "git"
     packages = TypedList(GitRepo)
 
+    _collection_attribute = 'packages'
+
     def initiate(self, session=None):
         pass
 
@@ -296,6 +298,8 @@ class SVNDistribution(VCSDistribution):
     _cmd = "svn"
     packages = TypedList(SVNRepo)
 
+    _collection_attribute = 'packages'
+
     def install_packages(self, session, use_version=True):
         raise NotImplementedError
 SVNRepo._distribution = SVNDistribution
diff --git a/reproman/distributions/venv.py b/reproman/distributions/venv.py
index f9059ce51..e017bdfcd 100644
--- a/reproman/distributions/venv.py
+++ b/reproman/distributions/venv.py
@@ -38,6 +38,8 @@ class VenvPackage(Package):
     location = attrib()
     editable = attrib(default=False)
     files = attrib(default=attr.Factory(list))
+    _diff_cmp_fields = ('name', )
+    _diff_fields = ('version', )
 
 
 @attr.s
@@ -45,6 +47,8 @@ class VenvEnvironment(SpecObject):
     path = attrib()
     python_version = attrib()
     packages = TypedList(VenvPackage)
+    _diff_cmp_fields = ('path', 'python_version')
+    _collection_attribute = 'packages'
 
 
 @attr.s
@@ -54,6 +58,7 @@ class VenvDistribution(Distribution):
     path = attrib()
     venv_version = attrib()
     environments = TypedList(VenvEnvironment)
+    _collection_attribute = 'environments'
 
     def initiate(self, _):
         return
diff --git a/reproman/interface/diff.py b/reproman/interface/diff.py
index 5ab4e2704..57a493cdd 100644
--- a/reproman/interface/diff.py
+++ b/reproman/interface/diff.py
@@ -20,6 +20,8 @@
 from ..distributions.debian import DebianDistribution
 from ..distributions.conda import CondaDistribution
 from ..distributions.vcs import GitDistribution, SVNDistribution
+from ..distributions.venv import VenvDistribution, VenvEnvironment
+from ..distributions.base import SpecDiff
 
 __docformat__ = 'restructuredtext'
 
@@ -81,55 +83,40 @@ def __call__(prov1, prov2, satisfies):
     @staticmethod
     def diff(env_1, env_2):
 
-        result = {'method': 'diff', 'distributions': []}
-
         # distribution type -> package type string
         supported_distributions = {
             DebianDistribution: 'Debian package', 
             CondaDistribution: 'Conda package', 
             GitDistribution: 'Git repository',
-            SVNDistribution: 'SVN repository'
+            SVNDistribution: 'SVN repository', 
+            VenvDistribution: 'Venv environment', 
         }
 
         env_1_dist_types = { d.__class__ for d in env_1.distributions }
         env_2_dist_types = { d.__class__ for d in env_2.distributions }
         all_dist_types = env_1_dist_types.union(env_2_dist_types)
 
+        diffs = []
+
         for dist_type in all_dist_types:
             if dist_type not in supported_distributions:
                 msg = 'diff doesn\'t know how to handle %s' % str(dist_type)
                 raise ValueError(msg)
-            dist_res = {'pkg_type': supported_distributions[dist_type], 
-                        'pkg_diffs': []}
             dist_1 = env_1.get_distribution(dist_type)
-            if dist_1:
-                pkgs_1 = {p._diff_cmp_id: p for p in dist_1.packages}
-            else:
-                pkgs_1 = {}
             dist_2 = env_2.get_distribution(dist_type)
-            if dist_2:
-                pkgs_2 = {p._diff_cmp_id: p for p in dist_2.packages}
-            else:
-                pkgs_2 = {}
-            dist_res['pkgs_1'] = pkgs_1
-            dist_res['pkgs_2'] = pkgs_2
-            pkgs_1_s = set(pkgs_1)
-            pkgs_2_s = set(pkgs_2)
-            dist_res['pkgs_only_1'] = pkgs_1_s - pkgs_2_s
-            dist_res['pkgs_only_2'] = pkgs_2_s - pkgs_1_s
-            for cmp_key in pkgs_1_s.intersection(pkgs_2_s):
-                package_1 = pkgs_1[cmp_key]
-                package_2 = pkgs_2[cmp_key]
-                if package_1._diff_vals != package_2._diff_vals:
-                    dist_res['pkg_diffs'].append((package_1, package_2))
-            result['distributions'].append(dist_res)
+            diffs.append({'diff': SpecDiff(dist_1, dist_2), 
+                          'pkg_type': supported_distributions[dist_type]})
+
+        diffs.append({'diff': SpecDiff(env_1.files, env_2.files), 
+                      'pkg_type': 'files'})
 
         files1 = set(env_1.files)
         files2 = set(env_2.files)
-        result['files_1_only'] = files1 - files2
-        result['files_2_only'] = files2 - files1
 
-        return result
+        return {'method': 'diff', 
+                'diffs': diffs, 
+                'files_1_only': files1 - files2, 
+                'files_2_only': files2 - files1}
 
     @staticmethod
     def satisfies(env_1, env_2):
@@ -180,39 +167,60 @@ def render_cmdline_diff(result):
 
         status = 0
 
-        for dist_res in result['distributions']:
+        for diff_d in result['diffs']:
 
-            if dist_res['pkgs_only_1'] or dist_res['pkgs_only_2']:
-                print(_make_plural(dist_res['pkg_type']) + ':')
+            diff = diff_d['diff']
+            pkg_type = diff_d['pkg_type']
 
-            if dist_res['pkgs_only_1']:
-                for cmp_key in sorted(dist_res['pkgs_only_1']):
-                    package = dist_res['pkgs_1'][cmp_key]
-                    print('< %s' % package.diff_identity_string)
-                status = 3
-            if dist_res['pkgs_only_1'] and dist_res['pkgs_only_2']:
-                print('---')
-            if dist_res['pkgs_only_2']:
-                for cmp_key in sorted(dist_res['pkgs_only_2']):
-                    package = dist_res['pkgs_2'][cmp_key]
-                    print('> %s' % package.diff_identity_string)
-                status = 3
+            if pkg_type == 'files':
+                files_diff = diff_d['diff']
+                continue
 
-            for (package_1, package_2) in dist_res['pkg_diffs']:
-                print('%s %s:' % (dist_res['pkg_type'], 
-                                  " ".join(package_1._diff_cmp_id)))
-                print('< %s' % package_1.diff_subidentity_string)
-                print('---')
-                print('> %s' % package_2.diff_subidentity_string)
+            if diff.a_only or diff.b_only:
+                print(_make_plural(pkg_type) + ':')
                 status = 3
-
-        if result['files_1_only'] or result['files_2_only']:
+            for pkg_diff in diff.a_only:
+                print('< %s' % pkg_diff.a.diff_identity_string)
+            if diff.a_only and diff.b_only:
+                print('---')
+            for pkg_diff in diff.b_only:
+                print('> %s' % pkg_diff.b.diff_identity_string)
+
+            for pkg_diff in diff.a_and_b:
+                if not hasattr(pkg_diff, 'collection'):
+                    if pkg_diff.diff_vals_a != pkg_diff.diff_vals_b:
+                        print('%s %s:' % (pkg_type, ' '.join(pkg_diff.diff_cmp_id)))
+                        print('< %s' % pkg_diff.a.diff_subidentity_string)
+                        print('---')
+                        print('> %s' % pkg_diff.b.diff_subidentity_string)
+                        status = 3
+                else:
+                    a_only = [ pd.a for pd in pkg_diff.a_only ]
+                    b_only = [ pd.b for pd in pkg_diff.b_only ]
+                    ab = [ pd for pd in pkg_diff.a_and_b 
+                            if pd.diff_vals_a != pd.diff_vals_b ]
+                    if a_only or b_only or ab:
+                        print('%s %s:' % (pkg_type, ' '.join(pkg_diff.diff_cmp_id)))
+                        for pkg in a_only:
+                            print('< %s' % pkg.diff_identity_string)
+                        for pd in ab:
+                            print('< %s %s' % (pd.a.diff_identity_string, pd.a.diff_subidentity_string))
+                        if (a_only and b_only) or ab:
+                            print('---')
+                        for pkg in b_only:
+                            print('> %s' % pkg.diff_identity_string)
+                        for pd in ab:
+                            print('> %s %s' % (pd.b.diff_identity_string, pd.b.diff_subidentity_string))
+
+        files_1_only = [ t[0] for t in files_diff.collection if t[1] is None ]
+        files_2_only = [ t[1] for t in files_diff.collection if t[0] is None ]
+        if files_1_only or files_2_only:
             print('Files:')
-            for fname in result['files_1_only']:
+            for fname in files_1_only:
                 print('< %s' % fname)
-            if result['files_1_only'] and result['files_2_only']:
+            if files_1_only and files_2_only:
                 print('---')
-            for fname in result['files_2_only']:
+            for fname in files_2_only:
                 print('> %s' % fname)
             status = 3
 
diff --git a/reproman/interface/tests/files/diff_1.yaml b/reproman/interface/tests/files/diff_1.yaml
index 08a098312..40c498f02 100644
--- a/reproman/interface/tests/files/diff_1.yaml
+++ b/reproman/interface/tests/files/diff_1.yaml
@@ -58,4 +58,53 @@ distributions:
   - path: /path/1/to/different/svn/commit
     revision: 12
     uuid: 95e4b738-84c7-154c-f082-34d40e21fdd4
+- name: venv
+  path: /usr/bin/virtualenv
+  venv_version: 15.1.0
+  environments:
+  - path: /home/user/venvs/test/both
+    python_version: 2.7.15+
+    packages:
+    - name: six
+      version: 1.12.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+  - path: /home/user/venvs/test/bth_2
+    python_version: 2.7.15+
+    packages:
+    - name: one_only
+      version: 1.12.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+    - name: version_mismatch
+      version: 1.1.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+  - path: /home/user/venvs/test/1_only
+    python_version: 2.7.15+
+    packages:
+    - name: funcsigs
+      version: 1.0.2
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/funcsigs-1.0.2.dist-info/METADATA
+    - name: rpaths
+      version: '0.13'
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/rpaths-0.13.dist-info/METADATA
 files: [/etc/a, /etc/b]
diff --git a/reproman/interface/tests/files/diff_2.yaml b/reproman/interface/tests/files/diff_2.yaml
index bed05c111..ff9939fc3 100644
--- a/reproman/interface/tests/files/diff_2.yaml
+++ b/reproman/interface/tests/files/diff_2.yaml
@@ -58,4 +58,53 @@ distributions:
   - path: /path/2/to/different/svn/commit
     revision: 14
     uuid: 95e4b738-84c7-154c-f082-34d40e21fdd4
+- name: venv
+  path: /usr/bin/virtualenv
+  venv_version: 15.1.0
+  environments:
+  - path: /home/user/venvs/test/both
+    python_version: 2.7.15+
+    packages:
+    - name: six
+      version: 1.12.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+  - path: /home/user/venvs/test/bth_2
+    python_version: 2.7.15+
+    packages:
+    - name: two_only
+      version: 1.12.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+    - name: version_mismatch
+      version: 1.2.0
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/six-1.12.0.dist-info/METADATA
+      - lib/python2.7/site-packages/six.pyc
+      - lib/python2.7/site-packages/six.py
+  - path: /home/user/venvs/test/2_only
+    python_version: 2.7.15+
+    packages:
+    - name: funcsigs
+      version: 1.0.2
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/funcsigs-1.0.2.dist-info/METADATA
+    - name: rpaths
+      version: '0.13'
+      local: true
+      location: /home/user/venvs/test/lib/python2.7/site-packages
+      files:
+      - lib/python2.7/site-packages/rpaths-0.13.dist-info/METADATA
 files: [/etc/c, /etc/b]
diff --git a/reproman/interface/tests/test_diff.py b/reproman/interface/tests/test_diff.py
index 6248031aa..1668f6d05 100644
--- a/reproman/interface/tests/test_diff.py
+++ b/reproman/interface/tests/test_diff.py
@@ -152,6 +152,18 @@ def test_diff_svn():
         assert_not_in('(/path/2/to/common/svn/repo)', outputs.out)
 
 
+def test_diff_venv():
+    with swallow_outputs() as outputs:
+        args = ['diff', diff_1_yaml, diff_2_yaml]
+        rv = main(args)
+        assert_equal(rv, 3)
+        assert_equal(outputs.err, '')
+        assert_in('Venv environments:', outputs.out)
+        assert_in('< /home/user/venvs/test/1_only 2.7.15+', outputs.out)
+        assert_in('> /home/user/venvs/test/2_only 2.7.15+', outputs.out)
+        assert_not_in('/home/user/venvs/test/both', outputs.out)
+
+
 def test_diff_satisfies_unsupported_distribution():
     # using subprocess.call() here because we're looking for a condition 
     # that raises an exception in main(), so it doesn't return and we