diff --git a/Source/ARTProtocolMessage.m b/Source/ARTProtocolMessage.m
index e36d0bb1c..589058b1b 100644
--- a/Source/ARTProtocolMessage.m
+++ b/Source/ARTProtocolMessage.m
@@ -35,6 +35,26 @@ - (NSString *)getConnectionKey {
     return _connectionKey;
 }
 
+- (NSString *)getSyncIdentifierAtIndex:(NSInteger)index {
+    if (!_channelSerial || [_channelSerial isEqualToString:@""] || index > 1) {
+        return @"";
+    }
+    NSArray *a = [_channelSerial componentsSeparatedByString:@":"];
+    return a.count == 2 ? a[index] : @"";
+}
+
+- (NSString *)getSyncSequenceId {
+    return [self getSyncIdentifierAtIndex:0];
+}
+
+- (NSString *)getSyncCursor {
+    return [self getSyncIdentifierAtIndex:1];
+}
+
+- (BOOL)isEndOfSync {
+    return [[self getSyncCursor] isEqualToString:@""];
+}
+
 - (NSString *)description {
     NSMutableString *description = [NSMutableString stringWithFormat:@"<%@: %p> {\n", self.class, self];
     [description appendFormat:@" count: %d,\n", self.count];
diff --git a/Source/ARTRealtimePresence.m b/Source/ARTRealtimePresence.m
index 7d47bd006..572dabf51 100644
--- a/Source/ARTRealtimePresence.m
+++ b/Source/ARTRealtimePresence.m
@@ -183,6 +183,10 @@ @implementation ARTRealtimePresenceInternal {
     NSMutableDictionary<NSString *, ARTPresenceMessage *> *_internalMembers; // RTP17h
     
     NSMutableDictionary<NSString *, ARTPresenceMessage *> *_beforeSyncMembers; // RTP19
+    
+    // RTP18a
+    NSString *_syncSequenceId;
+    NSMutableDictionary<NSString *, ARTPresenceMessage *> *_membersBackup;
 }
 
 - (instancetype)initWithChannel:(ARTRealtimeChannelInternal *)channel logger:(ARTInternalLog *)logger {
@@ -690,23 +694,8 @@ - (void)broadcast:(ARTPresenceMessage *)pm {
     [_eventEmitter emit:[ARTEvent newWithPresenceAction:pm.action] with:pm];
 }
 
-/*
- * Checks that a channelSerial is the final serial in a sequence of sync messages,
- * by checking that there is nothing after the colon - RTP18b, RTP18c
- */
-- (bool)isLastChannelSerial:(NSString *)channelSerial {
-    if (!channelSerial || [channelSerial isEqualToString:@""]) {
-        return true;
-    }
-    NSArray *a = [channelSerial componentsSeparatedByString:@":"];
-    if (a.count > 1 && ![[a objectAtIndex:1] isEqualToString:@""]) {
-        return false;
-    }
-    return true;
-}
-
 - (void)onAttached:(ARTProtocolMessage *)message {
-    [self startSync];
+    [self startSyncWithSequenceId:[message getSyncSequenceId]];
     if (!message.hasPresence) {
         // RTP1 - when an ATTACHED message is received without a HAS_PRESENCE flag, reset PresenceMap (also RTP19a)
         [self endSync];
@@ -748,8 +737,9 @@ - (void)onMessage:(ARTProtocolMessage *)message {
 }
 
 - (void)onSync:(ARTProtocolMessage *)message {
-    if (!self.syncInProgress) {
-        [self startSync];
+    NSString *sequenceId = [message getSyncSequenceId];
+    if (!self.syncInProgress || ![_syncSequenceId isEqualToString:sequenceId]) { // RTP18a
+        [self startSyncWithSequenceId:sequenceId];
     }
     else {
         ARTLogDebug(self.logger, @"RT:%p C:%p (%@) PresenceMap sync is in progress", _realtime, _channel, _channel.name);
@@ -757,8 +747,7 @@ - (void)onSync:(ARTProtocolMessage *)message {
 
     [self onMessage:message];
 
-    // TODO: RTP18a (previous in-flight sync should be discarded)
-    if ([self isLastChannelSerial:message.channelSerial]) { // RTP18b, RTP18c
+    if ([message isEndOfSync]) { // RTP18b, RTP18c
         [self endSync];
         ARTLogDebug(self.logger, @"RT:%p C:%p (%@) PresenceMap sync ended", _realtime, _channel, _channel.name);
     }
@@ -940,23 +929,30 @@ - (void)reset {
     _internalMembers = [NSMutableDictionary new];
 }
 
-- (void)startSync {
-    ARTLogDebug(_logger, @"%p PresenceMap sync started", self);
+- (void)startSyncWithSequenceId:(NSString *)sequenceId {
+    ARTLogDebug(_logger, @"%p PresenceMap sync started with sequence id = %@", self, sequenceId);
+    if (_syncSequenceId && sequenceId && ![_syncSequenceId isEqualToString:sequenceId]) {
+        _members = [_membersBackup mutableCopy]; // discard sync - RTP18a
+        ARTLogDebug(_logger, @"%p PresenceMap sync with sequence id = %@ was discarded", self, _syncSequenceId);
+    }
     _beforeSyncMembers = [_members mutableCopy];
+    _syncSequenceId = sequenceId;
+    _membersBackup = [_members mutableCopy];
     _syncState = ARTPresenceSyncStarted;
     [_syncEventEmitter emit:[ARTEvent newWithPresenceSyncState:_syncState] with:nil];
 }
 
 - (void)endSync {
-    ARTLogVerbose(_logger, @"%p PresenceMap sync ending", self);
+    ARTLogVerbose(_logger, @"%p PresenceMap sync ending with sequence id = %@", self, _syncSequenceId);
     [self cleanUpAbsentMembers];
     [self leaveMembersNotPresentInSync];
     _syncState = ARTPresenceSyncEnded;
     _beforeSyncMembers = nil;
-
+    _membersBackup = nil;
     [_syncEventEmitter emit:[ARTEvent newWithPresenceSyncState:ARTPresenceSyncEnded] with:[_members allValues]];
     [_syncEventEmitter off];
-    ARTLogDebug(_logger, @"%p PresenceMap sync ended", self);
+    ARTLogDebug(_logger, @"%p PresenceMap sync with sequence id = %@ is ended", self, _syncSequenceId);
+    _syncSequenceId = nil;
 }
 
 - (void)failsSync:(ARTErrorInfo *)error {
diff --git a/Source/PrivateHeaders/Ably/ARTProtocolMessage+Private.h b/Source/PrivateHeaders/Ably/ARTProtocolMessage+Private.h
index 0722be468..f6b03ed96 100644
--- a/Source/PrivateHeaders/Ably/ARTProtocolMessage+Private.h
+++ b/Source/PrivateHeaders/Ably/ARTProtocolMessage+Private.h
@@ -24,6 +24,10 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (BOOL)mergeFrom:(ARTProtocolMessage *)msg;
 
+- (NSString *)getSyncSequenceId;
+- (NSString *)getSyncCursor;
+- (BOOL)isEndOfSync;
+
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/Source/PrivateHeaders/Ably/ARTRealtimePresence+Private.h b/Source/PrivateHeaders/Ably/ARTRealtimePresence+Private.h
index e7a4b6c63..617c841c4 100644
--- a/Source/PrivateHeaders/Ably/ARTRealtimePresence+Private.h
+++ b/Source/PrivateHeaders/Ably/ARTRealtimePresence+Private.h
@@ -51,7 +51,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)processMember:(ARTPresenceMessage *)message;
 - (void)reset;
 
-- (void)startSync;
+- (void)startSyncWithSequenceId:(NSString *)sequenceId;
 - (void)endSync;
 - (void)failsSync:(ARTErrorInfo *)error;