diff --git a/src/class-tiny-source-base.php b/src/class-tiny-source-base.php
index 859b208..c6f3cff 100644
--- a/src/class-tiny-source-base.php
+++ b/src/class-tiny-source-base.php
@@ -240,6 +240,11 @@ protected function create_alternative_sources( $original_source_html ) {
 						$source_attr_parts[ $attr ] = $attr_value;
 					}
 				}
+			} else {
+				$sizes_value = $this->get_attribute_value( $original_source_html, 'sizes' );
+				if ( $sizes_value ) {
+					$source_attr_parts['sizes'] = $sizes_value;
+				}
 			}
 
 			$source_attr_parts['type'] = $mimetype;
diff --git a/test/unit/TinyPictureTest.php b/test/unit/TinyPictureTest.php
index 68ba295..95298d2 100644
--- a/test/unit/TinyPictureTest.php
+++ b/test/unit/TinyPictureTest.php
@@ -366,6 +366,21 @@ public function test_image_without_src() {
         $this->assertSame($expected, $output);
     }
 
+    /**
+     * sizes attribute on <img> also have to be set  to <source> elements
+     */
+    public function test_img_sizes_attribute_is_propagated_to_sources() {
+        $this->wp->createImage(1000, '2026/04', 'test-320w.webp');
+        $this->wp->createImage(1000, '2026/04', 'test-640w.webp');
+        $this->wp->createImage(1000, '2026/04', 'test.webp');
+
+        $input = '<img srcset="/wp-content/uploads/2026/04/test-320w.jpg 320w, /wp-content/uploads/2026/04/test-640w.jpg 640w" sizes="(max-width: 600px) 320px, 640px" src="/wp-content/uploads/2026/04/test.jpg">';
+        $expected = '<picture><source srcset="/wp-content/uploads/2026/04/test-320w.webp 320w, /wp-content/uploads/2026/04/test-640w.webp 640w" sizes="(max-width: 600px) 320px, 640px" type="image/webp" /><img srcset="/wp-content/uploads/2026/04/test-320w.jpg 320w, /wp-content/uploads/2026/04/test-640w.jpg 640w" sizes="(max-width: 600px) 320px, 640px" src="/wp-content/uploads/2026/04/test.jpg"></picture>';
+        $output = $this->tiny_picture->replace_sources($input);
+
+        $this->assertSame($expected, $output);
+    }
+
     /**
      * Images with only a srcset are valid and will be wrapped
      */