diff --git a/mps/code/event.c b/mps/code/event.c
index 0b4d16353b4..c16ac133bbe 100644
--- a/mps/code/event.c
+++ b/mps/code/event.c
@@ -80,6 +80,9 @@ Res EventInit(void)
 {
   Res res;
 
+  /* Ensure that no event can be larger than the maximum event size. */
+  AVER(EventBufferSIZE <= EventSizeMAX);
+
   /* Only if this is the first call. */
   if(!eventInited) { /* See .trans.log */
     AVER(EventNext == 0);
diff --git a/mps/code/event.h b/mps/code/event.h
index f089560cf4d..99373c1712f 100644
--- a/mps/code/event.h
+++ b/mps/code/event.h
@@ -40,16 +40,17 @@ extern Res EventFlush(void);
 extern char *EventNext, *EventLimit;
 extern Word EventKindControl;
 
-#define EVENT_BEGIN(name, size) \
+#define EVENT_BEGIN(name, structSize) \
   BEGIN \
     if(BS_IS_MEMBER(EventKindControl, ((Index)Event##name##Kind))) { \
       Event##name##Struct *_event; \
-      size_t _size = size_tAlignUp(size, MPS_PF_ALIGN); \
+      size_t _size = size_tAlignUp(structSize, MPS_PF_ALIGN); \
       if (_size > (size_t)(EventLimit - EventNext)) \
         EventFlush(); \
       AVER(_size <= (size_t)(EventLimit - EventNext)); \
       _event = (void *)EventNext; \
       _event->code = Event##name##Code; \
+      _event->size = (EventSize)_size; \
       _event->clock = mps_clock();
 
 #define EVENT_END(name, size) \
diff --git a/mps/code/eventcom.h b/mps/code/eventcom.h
index f0f4bfdcc5f..1db05d4c0fe 100644
--- a/mps/code/eventcom.h
+++ b/mps/code/eventcom.h
@@ -9,6 +9,7 @@
 #ifndef eventcom_h
 #define eventcom_h
 
+#include <limits.h>
 #include "mpmtypes.h" /* for Word */
 #include "eventdef.h"
 
@@ -17,6 +18,8 @@
 
 typedef unsigned short EventCode;
 typedef unsigned EventKind;
+typedef unsigned short EventSize;
+#define EventSizeMAX USHRT_MAX
 
 typedef Byte EventStringLen;
 
@@ -80,15 +83,18 @@ typedef unsigned EventFU;
 typedef EventStringStruct EventFS;
 typedef double EventFD;
 
-/* Common prefix for all event structures */
+/* Common prefix for all event structures.  The size field allows an event
+   reader to skip over events whose codes it does not recognise. */
 typedef struct EventAnyStruct {
   EventCode code;
+  EventSize size;
   Word clock;
 } EventAnyStruct;
 
 #define EVENT_STRUCT(X, name, _code, always, kind, count, format) \
   typedef struct Event##name##Struct { \
     EventCode code; \
+    EventSize size; \
     Word clock; \
     EVENT_STRUCT_FIELDS_##count format \
   } Event##name##Struct;