diff --git a/include/tidy.h b/include/tidy.h
index 7e98e9b..d0c05a3 100644
--- a/include/tidy.h
+++ b/include/tidy.h
@@ -714,6 +714,9 @@ TIDY_EXPORT TidyOptionType TIDY_CALL tidyOptGetType( TidyOption opt );
 
 /** Is Option read-only? Some options (mainly internal use only options) are
  ** read-only.
+ ** @deprecated This is no longer a valid test for the public API; instead
+ **   you should test an option's availability using `tidyOptGetCategory()`
+ **   against `TidyInternalCategory`. This API will be removed!
  ** @param opt An instance of a TidyOption to query.
  ** @result Returns `yes` or `no` depending on whether or not the specified
  **         option is read-only.