diff --git a/packages/lib/src/link/connectors.ts b/packages/lib/src/link/connectors.ts
index 7bd62609..c87b848a 100644
--- a/packages/lib/src/link/connectors.ts
+++ b/packages/lib/src/link/connectors.ts
@@ -171,10 +171,11 @@ export abstract class WebSocketBaseConnector<
 	_heartbeatId: ReturnType<typeof setInterval> | null = null;
 	_heartbeatInterval: number | null = null;
 	_lastReceivedSeq = undefined;
-	/** Serialised messages sent but not yet acknowledged by the other side */
-	_sendBuffer: { seq: number, text: string }[] = [];
+	/** Serialised messages sent but not yet acknowledged by the other side, with their UTF-8 size in bytes */
+	_sendBuffer: { seq: number, text: string, size: number }[] = [];
+	/** Total UTF-8 size in bytes of the messages in the send buffer */
 	_sendBufferSize = 0;
-	/** Session is ended if the unacknowledged messages exceed this many characters, 0 for no limit */
+	/** Session is ended if the unacknowledged messages exceed this many bytes, 0 for no limit */
 	maxSendBufferSize = 0;
 
 	_reset() {
@@ -225,7 +226,7 @@ export abstract class WebSocketBaseConnector<
 		for (let index = 0; index < this._sendBuffer.length; index++) {
 			if (this._sendBuffer[index].seq <= seq) {
 				dropCount += 1;
-				this._sendBufferSize -= this._sendBuffer[index].text.length;
+				this._sendBufferSize -= this._sendBuffer[index].size;
 			} else {
 				break;
 			}
@@ -334,8 +335,9 @@ export abstract class WebSocketBaseConnector<
 		}
 
 		const text = JSON.stringify(message);
-		this._sendBuffer.push({ seq: message.seq, text });
-		this._sendBufferSize += text.length;
+		const size = Buffer.byteLength(text, "utf8");
+		this._sendBuffer.push({ seq: message.seq, text, size });
+		this._sendBufferSize += size;
 		if (this.maxSendBufferSize && this._sendBufferSize > this.maxSendBufferSize) {
 			this._sendBufferOverflow();
 			return;
diff --git a/test/lib/link/connector.js b/test/lib/link/connector.js
index a19f3696..cc02fb03 100644
--- a/test/lib/link/connector.js
+++ b/test/lib/link/connector.js
@@ -13,9 +13,9 @@ describe("lib/link/connectors", function() {
 		describe("._dropSendBufferSeq()", function() {
 			beforeEach(function() {
 				testConnector._sendBuffer = [
-					{ seq: 1, text: "a" },
-					{ seq: 2, text: "bb" },
-					{ seq: 4, text: "cccc" },
+					{ seq: 1, text: "a", size: 1 },
+					{ seq: 2, text: "bb", size: 2 },
+					{ seq: 4, text: "cccc", size: 4 },
 				];
 				testConnector._sendBufferSize = 7;
 			});
@@ -64,10 +64,83 @@ describe("lib/link/connectors", function() {
 		});
 
 		describe("send buffer limit", function() {
+			const addr = new lib.Address(lib.Address.control, 1);
+			function textEvent(seq, text) {
+				return new lib.MessageEvent(seq, addr, addr, "TestEvent", { text });
+			}
+			function utf8Size(message) {
+				return Buffer.byteLength(JSON.stringify(message), "utf8");
+			}
+			function clearBuffer(connector) {
+				connector._state = "connected";
+				connector._sendBuffer = [];
+				connector._sendBufferSize = 0;
+				connector._socket.closeCalled = false;
+				connector._socket.sentMessages = [];
+			}
+
 			afterEach(function() {
 				testConnector.maxSendBufferSize = 0;
 				testConnector._closing = false;
 			});
+			it("should count buffered messages in UTF-8 bytes", function() {
+				// Extra bytes over the UTF-16 length for ASCII, 2 and 3 byte BMP characters and a surrogate pair
+				for (const [text, extraBytes] of [["ascii", 0], ["\u00e9", 1], ["\u20ac", 2], ["\u{1f600}", 2]]) {
+					clearBuffer(testConnector);
+					const event = textEvent(1, text);
+					testConnector.send(event);
+					const expected = JSON.stringify(event).length + extraBytes;
+					assert.equal(utf8Size(event), expected, text);
+					assert.equal(testConnector._sendBufferSize, expected, text);
+				}
+			});
+			it("should allow the limit to be reached exactly and close one byte over it", function() {
+				const event = textEvent(1, "\u20ac\u20ac\u20ac\u20ac");
+				clearBuffer(testConnector);
+				testConnector.maxSendBufferSize = utf8Size(event);
+				testConnector.send(event);
+				assert.equal(testConnector._sendBuffer.length, 1);
+				assert(!testConnector._socket.closeCalled, "Close was called on the socket");
+
+				clearBuffer(testConnector);
+				testConnector.maxSendBufferSize = utf8Size(event) - 1;
+				assert(JSON.stringify(event).length <= testConnector.maxSendBufferSize);
+				testConnector.send(event);
+				assert.equal(testConnector._sendBuffer.length, 0);
+				assert.equal(testConnector._sendBufferSize, 0);
+				assert(testConnector._socket.closeCalled, "Close was not called on the socket");
+				assert.equal(testConnector._socket.sentMessages.length, 0);
+			});
+			it("should subtract the bytes of acknowledged messages", function() {
+				const messages = [textEvent(1, "\u00e9"), textEvent(2, "\u20ac"), textEvent(3, "\u{1f600}")];
+				clearBuffer(testConnector);
+				for (const event of messages) {
+					testConnector.send(event);
+				}
+				assert.equal(testConnector._sendBufferSize, messages.reduce((sum, event) => sum + utf8Size(event), 0));
+				testConnector._dropSendBufferSeq(2);
+				assert.equal(testConnector._sendBuffer.length, 1);
+				assert.equal(testConnector._sendBufferSize, utf8Size(messages[2]));
+				testConnector._dropSendBufferSeq(3);
+				assert.equal(testConnector._sendBuffer.length, 0);
+				assert.equal(testConnector._sendBufferSize, 0);
+			});
+			it("should clear the byte count when the session is invalidated or reset", function() {
+				const connector = new lib.WebSocketClientConnector("url", 1);
+				connector._socket = new mock.MockSocket();
+				clearBuffer(connector);
+				connector.send(textEvent(1, "\u20ac"));
+				connector._invalidate();
+				assert.equal(connector._sendBuffer.length, 0);
+				assert.equal(connector._sendBufferSize, 0);
+
+				clearBuffer(connector);
+				connector.send(textEvent(1, "\u20ac"));
+				connector._socket = null;
+				connector._reset();
+				assert.equal(connector._sendBuffer.length, 0);
+				assert.equal(connector._sendBufferSize, 0);
+			});
 			it("should keep unacknowledged messages under the limit", function() {
 				testConnector._state = "connected";
 				testConnector._sendBuffer = [];