diff --git a/Tests/test_imagecms.py b/Tests/test_imagecms.py
index bf629fa79cc..a96b5f0e726 100644
--- a/Tests/test_imagecms.py
+++ b/Tests/test_imagecms.py
@@ -663,7 +663,8 @@ def test_auxiliary_channels_isolated() -> None:
 
 def test_long_modes() -> None:
     p = ImageCms.getOpenProfile("Tests/icc/sGrey-v2-nano.icc")
-    ImageCms.buildTransform(p, p, "ABCDEFGHI", "ABCDEFGHI")
+    with pytest.warns(DeprecationWarning):
+        ImageCms.buildTransform(p, p, "ABCDEFGHI", "ABCDEFGHI")
 
 
 @pytest.mark.parametrize("mode", ("RGB", "RGBA", "RGBX"))
@@ -684,3 +685,9 @@ def test_deprecation() -> None:
         assert ImageCms.VERSION == "1.0.0 pil"
     with pytest.warns(DeprecationWarning):
         assert isinstance(ImageCms.FLAGS, dict)
+
+    profile = ImageCmsProfile(ImageCms.createProfile("sRGB"))
+    with pytest.warns(DeprecationWarning):
+        ImageCms.ImageCmsTransform(profile, profile, "RGBA;16B", "RGB")
+    with pytest.warns(DeprecationWarning):
+        ImageCms.ImageCmsTransform(profile, profile, "RGB", "RGBA;16B")
diff --git a/docs/deprecations.rst b/docs/deprecations.rst
index b2cd968fee3..49a3c80cb32 100644
--- a/docs/deprecations.rst
+++ b/docs/deprecations.rst
@@ -115,6 +115,15 @@ Support for LibTIFF earlier than 4
 Support for LibTIFF earlier than version 4 has been deprecated.
 Upgrade to a newer version of LibTIFF instead.
 
+Non-image modes in ImageCms
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. deprecated:: 10.4.0
+
+The use in ImageCms of input modes and output modes that are not Pillow image
+modes has been deprecated. Defaulting to "L" or "1" if the mode cannot be
+mapped is also deprecated.
+
 Removed features
 ----------------
 
diff --git a/src/PIL/ImageCms.py b/src/PIL/ImageCms.py
index 5f5c5df54e7..cc130f2c3d7 100644
--- a/src/PIL/ImageCms.py
+++ b/src/PIL/ImageCms.py
@@ -299,6 +299,31 @@ def __init__(
         proof_intent: Intent = Intent.ABSOLUTE_COLORIMETRIC,
         flags: Flags = Flags.NONE,
     ):
+        supported_modes = (
+            "RGB",
+            "RGBA",
+            "RGBX",
+            "CMYK",
+            "I;16",
+            "I;16L",
+            "I;16B",
+            "YCbCr",
+            "LAB",
+            "L",
+            "1",
+        )
+        for mode in (input_mode, output_mode):
+            if mode not in supported_modes:
+                deprecate(
+                    mode,
+                    12,
+                    {
+                        "L;16": "I;16 or I;16L",
+                        "L:16B": "I;16B",
+                        "YCCA": "YCbCr",
+                        "YCC": "YCbCr",
+                    }.get(mode),
+                )
         if proof is None:
             self.transform = core.buildTransform(
                 input.profile, output.profile, input_mode, output_mode, intent, flags
diff --git a/src/_imagingcms.c b/src/_imagingcms.c
index 63d78f84daa..c47e3866ddc 100644
--- a/src/_imagingcms.c
+++ b/src/_imagingcms.c
@@ -226,13 +226,21 @@ findLCMStype(char *PILmode) {
     if (strcmp(PILmode, "CMYK") == 0) {
         return TYPE_CMYK_8;
     }
-    if (strcmp(PILmode, "L;16") == 0) {
+    if (
+        strcmp(PILmode, "I;16") == 0 ||
+        strcmp(PILmode, "I;16L") == 0 ||
+        strcmp(PILmode, "L;16") == 0
+    ) {
         return TYPE_GRAY_16;
     }
-    if (strcmp(PILmode, "L;16B") == 0) {
+    if (
+        strcmp(PILmode, "I;16B") == 0 ||
+        strcmp(PILmode, "L;16B") == 0
+    ) {
         return TYPE_GRAY_16_SE;
     }
     if (
+        strcmp(PILmode, "YCbCr") == 0 ||
         strcmp(PILmode, "YCCA") == 0 ||
         strcmp(PILmode, "YCC") == 0
     ) {
@@ -242,7 +250,7 @@ findLCMStype(char *PILmode) {
         // LabX equivalent like ALab, but not reversed -- no #define in lcms2
         return (COLORSPACE_SH(PT_LabV2) | CHANNELS_SH(3) | BYTES_SH(1) | EXTRA_SH(1));
     }
-    /* presume "L" by default */
+    /* presume "1" or "L" by default */
     return TYPE_GRAY_8;
 }