diff --git a/includes/rts/EventLogFormat.h b/includes/rts/EventLogFormat.h
index 67ce06afcd504001f43309a0e4c5bac3fe92d56b..925aec4ed117f0f01dee2895b341e3a26ae2eabd 100644
--- a/includes/rts/EventLogFormat.h
+++ b/includes/rts/EventLogFormat.h
@@ -140,20 +140,23 @@
 #define EVENT_SPARK_STEAL         39 /* (victim_cap)           */
 #define EVENT_SPARK_FIZZLE        40 /* ()                     */
 #define EVENT_SPARK_GC            41 /* ()                     */
+#define EVENT_INTERN_STRING       42 /* (string, id) {not used by ghc} */
 
 
-/* Range 42 - 59 is available for new events */
+/* Range 43 - 59 is available for new GHC and common events */
 
 /* Range 60 - 80 is used by eden for parallel tracing
  * see http://www.mathematik.uni-marburg.de/~eden/
  */
 
+/* Range 100 - 139 is reserved for Mercury */
+
 /*
  * The highest event code +1 that ghc itself emits. Note that some event
  * ranges higher than this are reserved but not currently emitted by ghc.
  * This must match the size of the EventDesc[] array in EventLog.c
  */
-#define NUM_EVENT_TAGS            42
+#define NUM_GHC_EVENT_TAGS        42
 
 #if 0  /* DEPRECATED EVENTS: */
 /* we don't actually need to record the thread, it's implicit */
diff --git a/rts/eventlog/EventLog.c b/rts/eventlog/EventLog.c
index c6ab86d7365f6a7885656036d78695019b4294c0..db0f3e4ad5771a9b8e803f3162e40ba18d7cf3d4 100644
--- a/rts/eventlog/EventLog.c
+++ b/rts/eventlog/EventLog.c
@@ -101,7 +101,7 @@ typedef struct _EventType {
   char *desc;     // Description
 } EventType;
 
-EventType eventTypes[NUM_EVENT_TAGS];
+EventType eventTypes[NUM_GHC_EVENT_TAGS];
 
 static void initEventsBuf(EventsBuf* eb, StgWord64 size, EventCapNo capno);
 static void resetEventsBuf(EventsBuf* eb);
@@ -200,7 +200,7 @@ initEventLogging(void)
                                         + 10 /* .eventlog */,
                                         "initEventLogging");
 
-    if (sizeof(EventDesc) / sizeof(char*) != NUM_EVENT_TAGS) {
+    if (sizeof(EventDesc) / sizeof(char*) != NUM_GHC_EVENT_TAGS) {
         barf("EventDesc array has the wrong number of elements");
     }
 
@@ -250,7 +250,7 @@ initEventLogging(void)
 
     // Mark beginning of event types in the header.
     postInt32(&eventBuf, EVENT_HET_BEGIN);
-    for (t = 0; t < NUM_EVENT_TAGS; ++t) {
+    for (t = 0; t < NUM_GHC_EVENT_TAGS; ++t) {
 
         eventTypes[t].etNum = t;
         eventTypes[t].desc = EventDesc[t];