diff --git a/Source/ARTRealtimeChannel.m b/Source/ARTRealtimeChannel.m
index e4510f3cf..5613b68a5 100644
--- a/Source/ARTRealtimeChannel.m
+++ b/Source/ARTRealtimeChannel.m
@@ -69,6 +69,10 @@ - (ARTRealtimeChannelState)state {
     return _internal.state;
 }
 
+- (ARTChannelProperties *)properties {
+    return _internal.properties;
+}
+
 - (ARTErrorInfo *)errorReason {
     return _internal.errorReason;
 }
@@ -1078,6 +1082,18 @@ - (ARTRealtimeChannelOptions *)getOptions_nosync {
     return (ARTRealtimeChannelOptions *)[self options_nosync];
 }
 
+- (ARTChannelProperties *)properties {
+    __block ARTChannelProperties *ret;
+    dispatch_sync(_queue, ^{
+        ret = [self properties_nosync];
+    });
+    return ret;
+}
+
+- (ARTChannelProperties *)properties_nosync {
+    return [[ARTChannelProperties alloc] initWithAttachSerial:_attachSerial channelSerial:_channelSerial];
+}
+
 - (void)setOptions:(ARTRealtimeChannelOptions *_Nullable)options callback:(nullable ARTCallback)callback {
     if (callback) {
         ARTCallback userCallback = callback;
@@ -1119,6 +1135,21 @@ - (void)setOptions_nosync:(ARTRealtimeChannelOptions *_Nullable)options callback
 
 @end
 
+#pragma mark - Channel Properties (RTL15)
+
+@implementation ARTChannelProperties
+
+- (instancetype)initWithAttachSerial:(NSString *)attachSerial channelSerial:(NSString *)channelSerial {
+    self = [super init];
+    if (self) {
+        _attachSerial = attachSerial;
+        _channelSerial = channelSerial;
+    }
+    return self;
+}
+
+@end
+
 #pragma mark - ARTEvent
 
 @implementation ARTEvent (ChannelEvent)
diff --git a/Source/PrivateHeaders/Ably/ARTRealtimeChannel+Private.h b/Source/PrivateHeaders/Ably/ARTRealtimeChannel+Private.h
index 15e9e0a0a..03dbbc572 100644
--- a/Source/PrivateHeaders/Ably/ARTRealtimeChannel+Private.h
+++ b/Source/PrivateHeaders/Ably/ARTRealtimeChannel+Private.h
@@ -34,6 +34,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (NSString * _Nullable)clientId_nosync;
 - (BOOL)canBeReattached;
 - (BOOL)shouldAttach;
+- (ARTChannelProperties *)properties_nosync;
 
 @property (readonly, weak, nonatomic) ARTRealtimeInternal *realtime; // weak because realtime owns self
 @property (readonly, nonatomic) ARTRestChannelInternal *restChannel;
@@ -95,4 +96,10 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+@interface ARTChannelProperties ()
+
+- (instancetype)initWithAttachSerial:(nullable NSString *)attachSerial channelSerial:(nullable NSString *)channelSerial;
+
+@end
+
 NS_ASSUME_NONNULL_END
diff --git a/Source/include/Ably/ARTRealtimeChannel.h b/Source/include/Ably/ARTRealtimeChannel.h
index 6a153d8aa..ecf14de4a 100644
--- a/Source/include/Ably/ARTRealtimeChannel.h
+++ b/Source/include/Ably/ARTRealtimeChannel.h
@@ -11,6 +11,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @class ARTRealtimePresence;
 @class ARTRealtimeChannelOptions;
+@class ARTChannelProperties;
 #if TARGET_OS_IPHONE
 @class ARTPushChannel;
 #endif
@@ -25,6 +26,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (readonly) ARTRealtimeChannelState state;
 
+/**
+ * An `ARTChannelProperties` object.
+ */
+@property (readonly) ARTChannelProperties *properties;
+
 /**
  * An `ARTErrorInfo` object describing the last error which occurred on the channel, if any.
  */
@@ -145,6 +151,18 @@ ART_EMBED_INTERFACE_EVENT_EMITTER(ARTChannelEvent, ARTChannelStateChange *)
 
 @end
 
+/**
+ * Describes the properties of the channel state.
+ */
+@interface ARTChannelProperties : NSObject
+/**
+ * Starts unset when a channel is instantiated, then updated with the `channelSerial` from each `ARTChannelEventAttached` event that matches the channel. Used as the value for `ARTRealtimeHistoryQuery.untilAttach`.
+ */
+@property (nonatomic, readonly, nullable) NSString *attachSerial; // CP2a
+@property (nonatomic, readonly, nullable) NSString *channelSerial; // CP2b
+
+@end
+
 /**
  * Enables messages to be published and subscribed to. Also enables historic messages to be retrieved and provides access to the `ARTRealtimePresence` object of a channel.
  * Also implements `ARTEventEmitter` interface and emits `ARTChannelEvent` events, where a `ARTChannelEvent` is either a `ARTRealtimeChannelState` or an `ARTChannelEvent.ARTChannelEventUpdate`.
diff --git a/Test/Tests/RealtimeClientChannelTests.swift b/Test/Tests/RealtimeClientChannelTests.swift
index 752278d35..3db6ca63d 100644
--- a/Test/Tests/RealtimeClientChannelTests.swift
+++ b/Test/Tests/RealtimeClientChannelTests.swift
@@ -134,7 +134,7 @@ private func testWithUntilAttach(_ untilAttach: Bool, for test: Test, channelNam
     let queryString = testHTTPExecutor.requests.last!.url!.query
 
     if query.untilAttach {
-        expect(queryString).to(contain("fromSerial=\(channel.internal.attachSerial!)"))
+        expect(queryString).to(contain("fromSerial=\(channel.properties.attachSerial!)"))
     } else {
         expect(queryString).toNot(contain("fromSerial"))
     }
@@ -4256,14 +4256,14 @@ class RealtimeClientChannelTests: XCTestCase {
                 expect(error).to(beNil())
                 let attachMessage = transport.protocolMessagesReceived.filter { $0.action == .attached }[0]
                 if attachMessage.channelSerial != nil {
-                    expect(attachMessage.channelSerial).to(equal(channel.internal.channelSerial))
+                    expect(attachMessage.channelSerial).to(equal(channel.properties.channelSerial))
                 }
                 partialDone()
                 
                 channel.subscribe { message in
                     let messageMessage = transport.protocolMessagesReceived.filter { $0.action == .message }[0]
                     if messageMessage.channelSerial != nil {
-                        expect(messageMessage.channelSerial).to(equal(channel.internal.channelSerial))
+                        expect(messageMessage.channelSerial).to(equal(channel.properties.channelSerial))
                     }
                     channel.presence.enterClient("client1", data: "Hey")
                     partialDone()
@@ -4271,7 +4271,7 @@ class RealtimeClientChannelTests: XCTestCase {
                 channel.presence.subscribe { presenceMessage in
                     let presenceMessage = transport.protocolMessagesReceived.filter { $0.action == .presence }[0]
                     if presenceMessage.channelSerial != nil {
-                        expect(presenceMessage.channelSerial).to(equal(channel.internal.channelSerial))
+                        expect(presenceMessage.channelSerial).to(equal(channel.properties.channelSerial))
                     }
                     partialDone()
                 }