diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1aef02f..a42ca96 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+* Added `Tinify::Format` constants for the media types `convert` accepts, including JPEG XL (`image/jxl`).
+
 ## 1.8.0
 
 * Update development dependencies.
diff --git a/lib/tinify.rb b/lib/tinify.rb
index 9fbc4a4..f1e7b8f 100644
--- a/lib/tinify.rb
+++ b/lib/tinify.rb
@@ -1,5 +1,6 @@
 require "tinify/version"
 require "tinify/error"
+require "tinify/format"
 
 require "tinify/client"
 require "tinify/result_meta"
diff --git a/lib/tinify/format.rb b/lib/tinify/format.rb
new file mode 100644
index 0000000..047592a
--- /dev/null
+++ b/lib/tinify/format.rb
@@ -0,0 +1,36 @@
+module Tinify
+  # Media types that images can be converted to.
+  #
+  # Use these constants with Source#convert:
+  #
+  #   Tinify.from_file("input.png")
+  #     .convert(type: Tinify::Format::JXL)
+  #     .to_file("output.jxl")
+  #
+  # Multiple types may be supplied, in which case the API returns the smallest
+  # result:
+  #
+  #   convert(type: [Tinify::Format::JXL, Tinify::Format::WEBP])
+  module Format
+    # WebP.
+    WEBP = "image/webp".freeze
+
+    # PNG.
+    PNG = "image/png".freeze
+
+    # JPEG.
+    JPEG = "image/jpeg".freeze
+
+    # JPEG, an alias of JPEG.
+    JPG = "image/jpg".freeze
+
+    # AVIF.
+    AVIF = "image/avif".freeze
+
+    # JPEG XL.
+    JXL = "image/jxl".freeze
+
+    # Wildcard, returns the smallest of the supported types.
+    ANY = "*/*".freeze
+  end
+end
diff --git a/test/tinify_format_test.rb b/test/tinify_format_test.rb
new file mode 100644
index 0000000..94420bf
--- /dev/null
+++ b/test/tinify_format_test.rb
@@ -0,0 +1,56 @@
+require File.expand_path("../helper", __FILE__)
+
+describe Tinify::Format do
+  describe "constants" do
+    it "should expose the supported media types" do
+      assert_equal "image/webp", Tinify::Format::WEBP
+      assert_equal "image/png", Tinify::Format::PNG
+      assert_equal "image/jpeg", Tinify::Format::JPEG
+      assert_equal "image/jpg", Tinify::Format::JPG
+      assert_equal "image/avif", Tinify::Format::AVIF
+      assert_equal "image/jxl", Tinify::Format::JXL
+      assert_equal "*/*", Tinify::Format::ANY
+    end
+  end
+
+  describe "convert" do
+    before do
+      Tinify.key = "valid"
+
+      stub_request(:post, "https://api.tinify.com/shrink")
+        .with(basic_auth: ['api', 'valid'])
+        .to_return(
+          status: 201,
+          headers: { Location: "https://api.tinify.com/some/location" },
+          body: '{}')
+    end
+
+    it "should serialize a single format" do
+      stub_request(:post, "https://api.tinify.com/some/location")
+        .with(
+          basic_auth: ['api', 'valid'],
+          body: '{"convert":{"type":"image/jxl"}}')
+        .to_return(
+          status: 200,
+          body: "converted file")
+
+      source = Tinify::Source.from_buffer("png file")
+        .convert(type: Tinify::Format::JXL)
+      assert_equal "converted file", source.to_buffer
+    end
+
+    it "should serialize multiple formats" do
+      stub_request(:post, "https://api.tinify.com/some/location")
+        .with(
+          basic_auth: ['api', 'valid'],
+          body: '{"convert":{"type":["image/jxl","image/webp"]}}')
+        .to_return(
+          status: 200,
+          body: "converted file")
+
+      source = Tinify::Source.from_buffer("png file")
+        .convert(type: [Tinify::Format::JXL, Tinify::Format::WEBP])
+      assert_equal "converted file", source.to_buffer
+    end
+  end
+end