diff --git a/src/debug.c b/src/debug.c
index 401a946..2fcc402 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -24,7 +24,7 @@ debug (int level, char *string)
   if (debugCond (level))
     {
       indent ();
-      printf ("DEBUG [%i]: %s\n", level, string);
+      fprintf (stderr, "DEBUG [%i]: %s\n", level, string);
     }
 #endif
 }
diff --git a/src/main.c b/src/main.c
index 4b4a045..88780d5 100644
--- a/src/main.c
+++ b/src/main.c
@@ -95,7 +95,7 @@ main (int argc, char **argv)
   if (arg_nullcheck (argtable) != 0)
     {
       /* NULL entries were detected, some allocations must have failed */
-      printf ("%s: insufficient memory\n", progname);
+      fprintf (stderr, "%s: insufficient memory\n", progname);
       exitcode = 1;
       goto exit;
     }
@@ -168,10 +168,10 @@ main (int argc, char **argv)
   /* Lutger-tries-to-test-with-broken-methods detector */
   if (clp->count > 0)
     {
-      printf ("For the time being, this method is not supported, \n");
-      printf ("as too many changes have been made to the normal \n");
-      printf ("matching logic, and CL simply isn't reliable in \nmany ");
-      printf ("ways. Try again in a few weeks.\n");
+      fprintf (stderr, "For the time being, this method is not supported, \n");
+      fprintf (stderr, "as too many changes have been made to the normal \n");
+      fprintf (stderr, "matching logic, and CL simply isn't reliable in \nmany ");
+      fprintf (stderr, "ways. Try again in a few weeks.\n");
       exit(0);
     }
 
@@ -182,7 +182,7 @@ main (int argc, char **argv)
       /* try to open */
       if (!freopen (outfile->filename[0], "w", stdout))
         {
-          printf("Could not create output file '%s'.\n", outfile->filename[0]);
+          fprintf(stderr, "Could not create output file '%s'.\n", outfile->filename[0]);
           exit(1);
         }
     }
@@ -194,7 +194,7 @@ main (int argc, char **argv)
 	{
           if (!freopen (infile->filename[0], "r", stdin))
 	    {
-	      printf("Could not open input file '%s'.\n", infile->filename[0]);
+	      fprintf(stderr, "Could not open input file '%s'.\n", infile->filename[0]);
 	      exit(1);
 	    }
 	}
diff --git a/src/modelchecker.c b/src/modelchecker.c
index 37b2225..94d5c4d 100644
--- a/src/modelchecker.c
+++ b/src/modelchecker.c
@@ -194,7 +194,7 @@ explorify (const System sys, const int run)
   rd = runPointerGet (sys, run);
   if (rd == NULL)
     {
-      printf ("ERROR: trying to progress completed run!\n");
+      fprintf (stderr, "ERROR: trying to progress completed run!\n");
       exit (1);
     }
 
@@ -354,7 +354,7 @@ traversePOR (const System sys)
 		  flag = block_clp (sys, phase);
 		  break;
 		default:
-		  printf ("Non existing clp switch.\n");
+		  fprintf (stderr, "Non existing clp switch.\n");
 		  exit (1);
 		}
 	    }
@@ -646,7 +646,7 @@ traversePOR2 (const System sys)
 		  flag = block_clp (sys, phase);
 		  break;
 		default:
-		  printf ("Non existing clp switch.\n");
+		  fprintf (stderr, "Non existing clp switch.\n");
 		  exit (1);
 		}
 	    }
@@ -829,7 +829,7 @@ traversePOR4 (const System sys)
 	      break;
 
 	    default:
-	      printf ("Encountered unknown event type %i.\n", rd->type);
+	      fprintf (stderr, "Encountered unknown event type %i.\n", rd->type);
 	      exit (1);
 	    }
 	}
@@ -877,6 +877,7 @@ propertyCheck (const System sys)
 	      if (claimev == -1)
 		{
 		  /* weird, should not occur */
+		  fprintf(stderr, "Violation, but cannot locate claim.\n");
 		  printf("A secrecy claim was supposed to be violated on term ");
 		  termPrint(scan->term);
 		  printf(" but we couldn't find the corresponding claim.\n");
@@ -988,7 +989,7 @@ claimViolationDetails (const System sys, const int run, const Roledef rd, const
 {
   if (rd->type != CLAIM)
     {
-      printf("Trying to determine details of something other than a claim!\n");
+      fprintf(stderr, "Trying to determine details of something other than a claim!\n");
       exit(-1);
     }