diff --git a/src/codec.ts b/src/codec.ts
index a396e70..b9414fd 100644
--- a/src/codec.ts
+++ b/src/codec.ts
@@ -337,6 +337,7 @@ export const extractVideoCodecString = (trackInfo: {
 	codec: VideoCodec | null;
 	codecDescription: Uint8Array | null;
 	colorSpace: VideoColorSpaceInit | null;
+	avcType: 1 | 3 | null;
 	avcCodecInfo: AvcDecoderConfigurationRecord | null;
 	hevcCodecInfo: HevcDecoderConfigurationRecord | null;
 	vp9CodecInfo: Vp9CodecInfo | null;
@@ -345,6 +346,8 @@ export const extractVideoCodecString = (trackInfo: {
 	const { codec, codecDescription, colorSpace, avcCodecInfo, hevcCodecInfo, vp9CodecInfo, av1CodecInfo } = trackInfo;
 
 	if (codec === 'avc') {
+		assert(trackInfo.avcType !== null);
+
 		if (avcCodecInfo) {
 			const bytes = new Uint8Array([
 				avcCodecInfo.avcProfileIndication,
@@ -352,14 +355,14 @@ export const extractVideoCodecString = (trackInfo: {
 				avcCodecInfo.avcLevelIndication,
 			]);
 
-			return `avc1.${bytesToHexString(bytes)}`;
+			return `avc${trackInfo.avcType}.${bytesToHexString(bytes)}`;
 		}
 
 		if (!codecDescription || codecDescription.byteLength < 4) {
 			throw new TypeError('AVC decoder description is not provided or is not at least 4 bytes long.');
 		}
 
-		return `avc1.${bytesToHexString(codecDescription.subarray(1, 4))}`;
+		return `avc${trackInfo.avcType}.${bytesToHexString(codecDescription.subarray(1, 4))}`;
 	} else if (codec === 'hevc') {
 		let generalProfileSpace: number;
 		let generalProfileIdc: number;
diff --git a/src/isobmff/isobmff-demuxer.ts b/src/isobmff/isobmff-demuxer.ts
index e1d047c..124c7b1 100644
--- a/src/isobmff/isobmff-demuxer.ts
+++ b/src/isobmff/isobmff-demuxer.ts
@@ -126,6 +126,7 @@ type InternalTrack = {
 		codec: VideoCodec | null;
 		codecDescription: Uint8Array | null;
 		colorSpace: VideoColorSpaceInit | null;
+		avcType: 1 | 3 | null;
 		avcCodecInfo: AvcDecoderConfigurationRecord | null;
 		hevcCodecInfo: HevcDecoderConfigurationRecord | null;
 		vp9CodecInfo: Vp9CodecInfo | null;
@@ -836,6 +837,7 @@ export class IsobmffDemuxer extends Demuxer {
 						codec: null,
 						codecDescription: null,
 						colorSpace: null,
+						avcType: null,
 						avcCodecInfo: null,
 						hevcCodecInfo: null,
 						vp9CodecInfo: null,
@@ -890,8 +892,9 @@ export class IsobmffDemuxer extends Demuxer {
 					const lowercaseBoxName = sampleBoxInfo.name.toLowerCase();
 
 					if (track.info.type === 'video') {
-						if (lowercaseBoxName === 'avc1') {
+						if (lowercaseBoxName === 'avc1' || lowercaseBoxName === 'avc3') {
 							track.info.codec = 'avc';
+							track.info.avcType = lowercaseBoxName === 'avc1' ? 1 : 3;
 						} else if (lowercaseBoxName === 'hvc1' || lowercaseBoxName === 'hev1') {
 							track.info.codec = 'hevc';
 						} else if (lowercaseBoxName === 'vp08') {
diff --git a/src/matroska/matroska-demuxer.ts b/src/matroska/matroska-demuxer.ts
index e964bef..1992205 100644
--- a/src/matroska/matroska-demuxer.ts
+++ b/src/matroska/matroska-demuxer.ts
@@ -2319,6 +2319,7 @@ class MatroskaVideoTrackBacking extends MatroskaTrackBacking implements InputVid
 					codec: this.internalTrack.info.codec,
 					codecDescription: this.internalTrack.info.codecDescription,
 					colorSpace: this.internalTrack.info.colorSpace,
+					avcType: 1, // We don't know better (or do we?) so just assume 'avc1'
 					avcCodecInfo: this.internalTrack.info.codec === 'avc' && firstPacket
 						? extractAvcDecoderConfigurationRecord(firstPacket.data)
 						: null,