diff --git a/libguile/guardians.c b/libguile/guardians.c
index d9cfc129b..3dd28a27c 100644
--- a/libguile/guardians.c
+++ b/libguile/guardians.c
@@ -176,12 +176,17 @@ void
 scm_guardian_zombify ()
 {
   guardian_t *g;
+
+  /* Note that new guardians may be stuck on the end of the live
+     guardian list as we run this loop.  As we move unmarked objects
+     to the zombie list and mark them, we may find some guarded
+     guardians.  The guardian mark function will stick them on the end
+     of this list, so they'll be processed properly.  */
   for (g = first_live_guardian; g; g = g->next)
     {
       /* Loop through the live list and
 	 1. move unmarked objects to the zombies tconc
-	 2. mark the live tconc.
-      */
+	 2. mark the live tconc.  */
       SCM tconc_tail = g->live.tail;
       SCM prev_pair = SCM_BOOL_F;
       SCM pair = g->live.head;
@@ -189,6 +194,21 @@ scm_guardian_zombify ()
 	{
 	  SCM next_pair = SCM_CDR (pair);
 
+	  /* Since next_pair is never assigned to, and since pair is
+	     always set to next_pair at the end of the loop, we know
+	     that this loop always reaches every pair in the tconc
+	     once, except the tail.
+
+	     At each iteration, we either throw the pair out of the
+	     live list, or we mark it.  And when we edit a pair's CDR,
+	     we make sure to preserve its mark bit.  So when this loop
+	     terminates, all pairs left in the live tconc (except the
+	     tail) have been marked.
+
+	     Whenever we throw out a pair, we always relink the tconc,
+	     so the tconc list structure is valid at the end of every
+	     iteration.  */
+
 	  if (SCM_NMARKEDP (SCM_CAR (pair)))
 	    {
 	      /* got you, zombie! */
@@ -197,28 +217,29 @@ scm_guardian_zombify ()
 	      if (SCM_FALSEP (prev_pair))
 		g->live.head = next_pair;
 	      else
-                /* mark previous pair */
-		SCM_SETCDR (prev_pair, next_pair | 1);
+		{
+		  SCM_SETCDR (prev_pair, next_pair);
+		  /* Don't lose the mark bit!  */
+		  SCM_SETGCMARK (prev_pair);
+		}
 
 	      /* to the zombie list! */
 	      TCONC_IN (g->zombies, SCM_CAR (pair), pair);
 	    }
 	  else
 	    {
-	      if (SCM_NFALSEP (prev_pair))
-                /* mark previous pair */
-		SCM_SETCDR (prev_pair, pair | 1);
+	      SCM_SETGCMARK (pair);
 	      prev_pair = pair;
 	    }
 
 	  pair = next_pair;
 	}
-      if (SCM_NFALSEP (prev_pair))
-	/* mark previous pair */
-	SCM_SETCDR (prev_pair, pair | 1);
-      /* mark live list tail */
-      SCM_SETOR_CDR (tconc_tail, 1);
       
+      /* Mark the tconc's tail cell.  All the other pairs in the tconc
+         are already marked.  */
+      SCM_SETGCMARK (tconc_tail);
+
+      /* Bring the zombies back from the dead.  */
       scm_gc_mark (g->zombies.head);
     }
 }