diff --git a/gc-notes.txt b/gc-notes.txt
index 6a96ca5d..6649d245 100644
--- a/gc-notes.txt
+++ b/gc-notes.txt
@@ -15,6 +15,9 @@ TODO:
   - add mutex type, and associated functions from SRFI-18
     when allocating a mutex, probably should do it on thread since by definition these are
     shared among multiple threads
+  - may be able to free mutex using mutex_destroy from within gc_sweep.
+    unfortunately it requires type checking each object before free, which is not ideal
+
   - start making core stuff thread safe. for example, test.scm sometimes
     crashes, I think printing out result from (read)
   - assume I/O and eval both have threading issues
diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index edadf27f..66ba61b0 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -206,6 +206,7 @@ typedef long tag_type;
 #define cvar_tag 16
 #define vector_tag 17
 #define macro_tag 18
+#define mutex_tag 19
 
 #define nil NULL
 #define eq(x,y) (x == y)
@@ -238,6 +239,11 @@ typedef struct {gc_header_type hdr; tag_type tag; object *pvar;} cvar_type;
 typedef cvar_type *cvar;
 #define make_cvar(n,v) cvar_type n; n.hdr.mark = gc_color_red; n.hdr.grayed = 0; n.tag = cvar_tag; n.pvar = v;
 
+// TODO: mutex type
+// thinking about maybe using cvar_type with a mutex tag
+// add an alloc_mutex macro/function, because these will only go on the heap
+
+
 /* Define boolean type. */
 typedef struct {gc_header_type hdr; const tag_type tag; const char *pname;} boolean_type;
 typedef boolean_type *boolean;