diff --git a/setup.cfg b/setup.cfg
index ce5f41f..d097611 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = surround-to-eac3
-version = 0.3.0
+version = 0.3.1
 author = Jonathan Rampersad
 author_email = jonathan@jono-rams.work
 description = A CLI tool to transcode 5.1 audio in video files to E-AC3.
diff --git a/src/surround_to_eac3/main.py b/src/surround_to_eac3/main.py
index 531c4e7..598c086 100644
--- a/src/surround_to_eac3/main.py
+++ b/src/surround_to_eac3/main.py
@@ -107,6 +107,12 @@ def process_file_with_ffmpeg(
         output_audio_stream_ffmpeg_idx += 1
     
     ffmpeg_cmd.extend(map_operations)
+
+    if final_output_filepath.lower().endswith('.mkv'):
+        ffmpeg_cmd.extend(['-f', 'matroska'])
+    elif final_output_filepath.lower().endswith('.mp4'):
+        ffmpeg_cmd.extend(['-f', 'mp4'])
+
     ffmpeg_cmd.extend(["-y", temp_output_filepath])
 
     logs.append(f"    ⚙️ Processing: '{base_filename}' -> '{output_filename}'")