diff --git a/crates/engine/src/commands/render_queue.rs b/crates/engine/src/commands/render_queue.rs
index 66fa0e2d..8abc855c 100644
--- a/crates/engine/src/commands/render_queue.rs
+++ b/crates/engine/src/commands/render_queue.rs
@@ -219,6 +219,9 @@ fn apply_output(om: &mut OutputModule, templates: &RenderTemplates, roi: Option<
         let f = OutputFormat::from_name(f).ok_or_else(|| bad(cmd, FORMATS))?;
         om.set_format(f);
     }
+    // Before Channels: the WebM codec decides whether alpha can be written, so an explicit
+    // `channels: rgba` with `webmCodec: av1` is refused rather than dropped (#166).
+    any |= apply_codec_options(om, p, cmd)?;
     if let Some(c) = str_p(p, "channels") {
         om.channels = match c.to_ascii_lowercase().replace([' ', '+'], "").as_str() {
             "rgb" => Channels::Rgb,
@@ -227,7 +230,11 @@ fn apply_output(om: &mut OutputModule, templates: &RenderTemplates, roi: Option<
             _ => return Err(bad(cmd, "channels: rgb|rgba|alpha")),
         };
         if om.channels == Channels::Rgba && !om.supports_alpha() {
-            return Err(bad(cmd, format!("{} has no alpha channel", om.format.label())));
+            let what = match om.format {
+                OutputFormat::WebM => format!("{} WebM has no alpha channel (VP9 WebM has)", om.webm_codec.label()),
+                f => format!("{} has no alpha channel", f.label()),
+            };
+            return Err(bad(cmd, what));
         }
     }
     if let Some(c) = str_p(p, "color").or(str_p(p, "alphaMode")) {
@@ -237,7 +244,6 @@ fn apply_output(om: &mut OutputModule, templates: &RenderTemplates, roi: Option<
             _ => return Err(bad(cmd, "color: straight|premultiplied")),
         };
     }
-    any |= apply_codec_options(om, p, cmd)?;
     if let Some(q) = f_p(p, "quality") {
         om.quality = q.clamp(1.0, 100.0) as u8;
     }
diff --git a/crates/engine/src/tests_codec_options.rs b/crates/engine/src/tests_codec_options.rs
index 80703063..8adabba5 100644
--- a/crates/engine/src/tests_codec_options.rs
+++ b/crates/engine/src/tests_codec_options.rs
@@ -110,6 +110,25 @@ fn output_to_keeps_a_format_that_writes_the_extension() {
     assert_eq!((&r["output"]["format"], &r["outputModuleSummary"]), (&json!("Hevc"), &a["outputModuleSummary"]), "{r}");
 }
 
+/// An explicit RGB + Alpha request with the AV1 WebM codec (which has no alpha) is refused, not
+/// silently rendered opaque (#166). Without `channels`, AV1 WebM renders RGB as before.
+#[test]
+fn av1_webm_refuses_requested_alpha() {
+    let mut s = session();
+    let n = s.project.render_queue.len();
+    let e = s.execute("renderQueue.add", json!({"format": "webm", "webmCodec": "av1", "channels": "rgba"})).unwrap_err().to_string();
+    assert!(e.contains("AV1 WebM has no alpha channel"), "{e}");
+    assert_eq!(s.project.render_queue.len(), n, "nothing queued");
+    let a = s.execute("renderQueue.add", json!({"format": "webm", "channels": "rgba"})).unwrap();
+    let e = s.execute("renderQueue.setOutputModule", json!({"item": a["item"], "webmCodec": "av1", "channels": "rgba"})).unwrap_err().to_string();
+    assert!(e.contains("AV1 WebM has no alpha channel"), "{e}");
+    let om = &s.project.render_queue.last().unwrap().output;
+    assert_eq!((om.webm_codec, om.channels), (WebmVideoCodec::Vp9, Channels::Rgba), "unchanged");
+    s.execute("renderQueue.add", json!({"format": "webm", "webmCodec": "av1"})).unwrap();
+    let om = &s.project.render_queue.last().unwrap().output;
+    assert_eq!((om.webm_codec, om.channels), (WebmVideoCodec::Av1, Channels::Rgb));
+}
+
 /// A `comp` that names no composition says so, rather than "no active composition" (#155).
 #[test]
 fn an_unknown_comp_is_named_in_the_error() {