diff --git a/cf_xarray/accessor.py b/cf_xarray/accessor.py
index ed0c216d..fe828099 100644
--- a/cf_xarray/accessor.py
+++ b/cf_xarray/accessor.py
@@ -2039,6 +2039,12 @@ def keys(self) -> set[Hashable]:
         varnames.extend(list(self.cell_measures))
         varnames.extend(list(self.standard_names))
         varnames.extend(list(self.cf_roles))
+        custom_criteria = ChainMap(*OPTIONS["custom_criteria"])
+        varnames.extend(
+            key
+            for key in custom_criteria
+            if _get_custom_criteria(self._obj, key, custom_criteria)
+        )
         if isinstance(self._obj, xr.Dataset):
             varnames.extend(list(self.grid_mapping_names))
         else:
diff --git a/cf_xarray/tests/test_accessor.py b/cf_xarray/tests/test_accessor.py
index 6657be46..cbd9338a 100644
--- a/cf_xarray/tests/test_accessor.py
+++ b/cf_xarray/tests/test_accessor.py
@@ -2174,6 +2174,20 @@ def test_custom_criteria() -> None:
         assert_identical(ds.cf["temp"], ds["temperature"])
 
 
+def test_custom_criteria_membership() -> None:
+    custom_criteria = {
+        "salt": {"standard_name": "salinity"},
+        "ssh": {"standard_name": "sea_surface_height"},
+    }
+    ds = xr.Dataset({"salinity": ("time", [34.5, 35.0], {"standard_name": "salinity"})})
+
+    with cf_xarray.set_options(custom_criteria=custom_criteria):
+        assert "salt" in ds.cf.keys()
+        assert "salt" in ds.cf
+        assert "ssh" not in ds.cf.keys()
+        assert "ssh" not in ds.cf
+
+
 @requires_regex
 def test_regex_match():
     # test that having a global regex expression flag later in the expression will work if