diff --git a/README.md b/README.md
index aafaea7..8c79ba6 100644
--- a/README.md
+++ b/README.md
@@ -59,7 +59,7 @@ Certain values can be set via environment variables, using the `-e` parameter on
 * __OUTPUT_TEMPLATE_CHANNEL__: The template for the filenames of the downloaded videos when downloaded as a channel. Defaults to `%(channel)s/%(title)s.%(ext)s`. When empty, then `OUTPUT_TEMPLATE` is used.
 * __YTDL_OPTIONS__: Additional options to pass to yt-dlp in JSON format. [See available options here](https://github.com/yt-dlp/yt-dlp/blob/master/yt_dlp/YoutubeDL.py#L222). They roughly correspond to command-line options, though some do not have exact equivalents here. For example, `--recode-video` has to be specified via `postprocessors`. Also note that dashes are replaced with underscores. You may find [this script](https://github.com/yt-dlp/yt-dlp/blob/master/devscripts/cli_to_api.py) helpful for converting from command-line options to `YTDL_OPTIONS`.
 * __YTDL_OPTIONS_FILE__: A path to a JSON file that will be loaded and used for populating `YTDL_OPTIONS` above. Please note that if both `YTDL_OPTIONS_FILE` and `YTDL_OPTIONS` are specified, the options in `YTDL_OPTIONS` take precedence. The file will be monitored for changes and reloaded automatically when changes are detected.
-* UI format __Captions__: Downloads subtitles/captions only (no media). Subtitle format, language, and source preference are configurable from Advanced Options (defaults: `ass`, `en`, `prefer_manual`).
+* UI format __Captions__: Downloads subtitles/captions only (no media). Subtitle format, language, and source preference are configurable from Advanced Options (defaults: `srt`, `en`, `prefer_manual`).
 
 ### 🌐 Web Server & URLs
 
diff --git a/app/dl_formats.py b/app/dl_formats.py
index 43384f0..867ceee 100644
--- a/app/dl_formats.py
+++ b/app/dl_formats.py
@@ -70,7 +70,7 @@ def get_opts(
     format: str,
     quality: str,
     ytdl_opts: dict,
-    subtitle_format: str = "ass",
+    subtitle_format: str = "srt",
     subtitle_language: str = "en",
     subtitle_mode: str = "prefer_manual",
 ) -> dict:
@@ -124,7 +124,7 @@ def get_opts(
         mode = _normalize_caption_mode(subtitle_mode)
         language = _normalize_subtitle_language(subtitle_language)
         opts["skip_download"] = True
-        opts["subtitlesformat"] = subtitle_format or "ass"
+        opts["subtitlesformat"] = subtitle_format or "srt"
         if mode == "manual_only":
             opts["writesubtitles"] = True
             opts["writeautomaticsub"] = False
diff --git a/app/main.py b/app/main.py
index 16865c6..c28c32c 100644
--- a/app/main.py
+++ b/app/main.py
@@ -264,7 +264,7 @@ async def add(request):
     if chapter_template is None:
         chapter_template = config.OUTPUT_TEMPLATE_CHAPTER
     if subtitle_format is None:
-        subtitle_format = 'ass'
+        subtitle_format = 'srt'
     if subtitle_language is None:
         subtitle_language = 'en'
     if subtitle_mode is None:
diff --git a/app/ytdl.py b/app/ytdl.py
index 72ee0bc..f7924de 100644
--- a/app/ytdl.py
+++ b/app/ytdl.py
@@ -100,7 +100,7 @@ class DownloadInfo:
         playlist_item_limit,
         split_by_chapters,
         chapter_template,
-        subtitle_format="ass",
+        subtitle_format="srt",
         subtitle_language="en",
         subtitle_mode="prefer_manual",
     ):
@@ -139,7 +139,7 @@ class Download:
             format,
             quality,
             ytdl_opts,
-            subtitle_format=getattr(info, 'subtitle_format', 'ass'),
+            subtitle_format=getattr(info, 'subtitle_format', 'srt'),
             subtitle_language=getattr(info, 'subtitle_language', 'en'),
             subtitle_mode=getattr(info, 'subtitle_mode', 'prefer_manual'),
         )
@@ -701,7 +701,7 @@ class DownloadQueue:
         auto_start=True,
         split_by_chapters=False,
         chapter_template=None,
-        subtitle_format="ass",
+        subtitle_format="srt",
         subtitle_language="en",
         subtitle_mode="prefer_manual",
         already=None,
diff --git a/ui/src/app/app.ts b/ui/src/app/app.ts
index 7fc4a23..d7651d9 100644
--- a/ui/src/app/app.ts
+++ b/ui/src/app/app.ts
@@ -101,10 +101,9 @@ export class App implements AfterViewInit, OnInit {
   faClock = faClock;
   faTachometerAlt = faTachometerAlt;
   subtitleFormats = [
-    { id: 'ass', text: 'ASS' },
-    { id: 'vtt', text: 'VTT' },
     { id: 'srt', text: 'SRT' },
-    { id: 'ttml', text: 'TTML' },
+    { id: 'vtt', text: 'VTT' },
+    { id: 'ttml', text: 'TTML' }
   ];
   subtitleLanguages = [
     { id: 'en', text: 'English' },
@@ -135,9 +134,13 @@ export class App implements AfterViewInit, OnInit {
     this.splitByChapters = this.cookieService.get('metube_split_chapters') === 'true';
     // Will be set from backend configuration, use empty string as placeholder
     this.chapterTemplate = this.cookieService.get('metube_chapter_template') || '';
-    this.subtitleFormat = this.cookieService.get('metube_subtitle_format') || 'ass';
+    this.subtitleFormat = this.cookieService.get('metube_subtitle_format') || 'srt';
     this.subtitleLanguage = this.cookieService.get('metube_subtitle_language') || 'en';
     this.subtitleMode = this.cookieService.get('metube_subtitle_mode') || 'prefer_manual';
+    const allowedSubtitleFormats = new Set(this.subtitleFormats.map(fmt => fmt.id));
+    if (!allowedSubtitleFormats.has(this.subtitleFormat)) {
+      this.subtitleFormat = 'srt';
+    }
 
     this.activeTheme = this.getPreferredTheme(this.cookieService);
 
diff --git a/ui/src/app/services/downloads.service.ts b/ui/src/app/services/downloads.service.ts
index ac21bc5..52690c8 100644
--- a/ui/src/app/services/downloads.service.ts
+++ b/ui/src/app/services/downloads.service.ts
@@ -180,7 +180,7 @@ export class DownloadsService {
     const defaultAutoStart = true;
     const defaultSplitByChapters = false;
     const defaultChapterTemplate = this.configuration['OUTPUT_TEMPLATE_CHAPTER'];
-    const defaultSubtitleFormat = 'ass';
+    const defaultSubtitleFormat = 'srt';
     const defaultSubtitleLanguage = 'en';
     const defaultSubtitleMode = 'prefer_manual';