diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3d7a5531..8682083e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,7 @@ The format is based on [Keep a Changelog].
   calling major-mode. Also includes setting for `indent-tabs-mode` ([#286]).
 * [Formatter scripts](scripts/formatters) will now work on Windows if Emacs
   can find the executable defined in the shebang.
+* `.heex` files are now properly formatted ([#304]).
 
 ### Internal
 * Major internal refactoring has occurred to make it possible to write
@@ -50,6 +51,7 @@ The format is based on [Keep a Changelog].
 [#285]: https://github.com/radian-software/apheleia/issues/285
 [#290]: https://github.com/radian-software/apheleia/pull/290
 [#302]: https://github.com/radian-software/apheleia/issues/302
+[#304]: https://github.com/radian-software/apheleia/pull/304
 
 ## 4.1 (released 2024-02-25)
 ### Enhancements
diff --git a/apheleia-formatters.el b/apheleia-formatters.el
index 6257e093..0da660b7 100644
--- a/apheleia-formatters.el
+++ b/apheleia-formatters.el
@@ -91,7 +91,8 @@
     (ktlint . ("ktlint" "--log-level=none" "--stdin" "-F" "-"))
     (latexindent . ("latexindent" "--logfile=/dev/null"))
     (mix-format . ("apheleia-from-project-root"
-                   ".formatter.exs" "mix" "format" "-"))
+                   ".formatter.exs" "mix" "format"
+		   "--stdin-filename" filepath "-"))
     (nixfmt . ("nixfmt"))
     (ocamlformat . ("ocamlformat" "-" "--name" filepath
                     "--enable-outside-detected-project"))
@@ -320,6 +321,7 @@ rather than using this system."
     (graphql-mode . prettier-graphql)
     (haskell-mode . brittany)
     (hcl-mode . hclfmt)
+    (heex-ts-mode . mix-format)
     (html-mode . prettier-html)
     (html-ts-mode . prettier-html)
     (java-mode . google-java-format)
diff --git a/test/formatters/installers/mix-format.bash b/test/formatters/installers/mix-format.bash
index f09283d4..427e7ca9 100644
--- a/test/formatters/installers/mix-format.bash
+++ b/test/formatters/installers/mix-format.bash
@@ -1 +1,3 @@
+add-apt-repository ppa:rabbitmq/rabbitmq-erlang
+apt-get update -y
 apt-get install -y elixir
diff --git a/test/formatters/samplecode/mix-format/in.erl b/test/formatters/samplecode/mix-format/in.ex
similarity index 100%
rename from test/formatters/samplecode/mix-format/in.erl
rename to test/formatters/samplecode/mix-format/in.ex
diff --git a/test/formatters/samplecode/mix-format/in.html.heex b/test/formatters/samplecode/mix-format/in.html.heex
new file mode 100644
index 00000000..a5e7683b
--- /dev/null
+++ b/test/formatters/samplecode/mix-format/in.html.heex
@@ -0,0 +1,6 @@
+<.component   id={  @a  }>
+  <:slot attr={@b}
+foo="bar">
+    <%= @c %>
+      </:slot>
+         </.component>
diff --git a/test/formatters/samplecode/mix-format/out.erl b/test/formatters/samplecode/mix-format/out.ex
similarity index 100%
rename from test/formatters/samplecode/mix-format/out.erl
rename to test/formatters/samplecode/mix-format/out.ex
diff --git a/test/formatters/samplecode/mix-format/out.html.heex b/test/formatters/samplecode/mix-format/out.html.heex
new file mode 100644
index 00000000..0fe0025d
--- /dev/null
+++ b/test/formatters/samplecode/mix-format/out.html.heex
@@ -0,0 +1,5 @@
+<.component id={@a}>
+  <:slot attr={@b} foo="bar">
+    <%= @c %>
+  </:slot>
+</.component>