diff --git a/js/script.js b/js/script.js
index 1c92b7d..bf1249e 100644
--- a/js/script.js
+++ b/js/script.js
@@ -155,6 +155,18 @@ const ConversionFunctions = {
 };
 settings.conversionFunction = ConversionFunctions.horizontal1bit;
 
+// Bytes per output value for each conversion function, matching the
+// hexWidth passed to writer.push() in that function (hexWidth / 2).
+// Needed by downloadBinFile() to reconstruct the true byte stream instead
+// of truncating multi-byte values (565/888) down to a single byte.
+const CONVERSION_BYTES_PER_VALUE = new Map([
+  [ConversionFunctions.horizontal1bit, 1],
+  [ConversionFunctions.vertical1bit, 1],
+  [ConversionFunctions.horizontal565, 2],
+  [ConversionFunctions.horizontal888, 4],
+  [ConversionFunctions.horizontalAlpha, 1],
+]);
+
 // An images collection with helper methods
 function Images() {
   const collection = [];
@@ -983,20 +995,37 @@ function copyOutput() {
   navigator.clipboard.writeText(output.value);
 }
 
+// Rebuilds the true byte stream for the current images/conversion mode by
+// re-splitting each formatted output value (which may be 1, 2 or 4 bytes
+// wide, depending on drawMode) back into its individual bytes, most-
+// significant first. Exposed standalone so it can be exercised directly in
+// tests without going through a real browser file download.
+function computeBinData() {
+  const bytesPerValue = CONVERSION_BYTES_PER_VALUE.get(settings.conversionFunction) || 1;
+  const raw = [];
+  images.each((image) => {
+    const values = imageToString(image)
+      .split(',')
+      .map((s) => s.trim())
+      .filter(Boolean)
+      .map((token) => parseInt(token, 16));
+    values.forEach((value) => {
+      // Split back into individual bytes, most-significant first, matching
+      // the left-to-right digit order produced by toString(16).padStart().
+      for (let shift = (bytesPerValue - 1) * 8; shift >= 0; shift -= 8) {
+        // eslint-disable-next-line no-bitwise
+        raw.push((value >> shift) & 0xff);
+      }
+    });
+  });
+  return new Uint8Array(raw);
+}
+
 // eslint-disable-next-line no-unused-vars
 function downloadBinFile() {
   if (!checkImagesAvailable()) return;
 
-  let raw = [];
-  images.each((image) => {
-    const data = imageToString(image)
-      .split(',')
-      .map((s) => s.trim())
-      .filter(Boolean)
-      .map((byte) => parseInt(byte, 16));
-    raw = raw.concat(data);
-  });
-  const data = new Uint8Array(raw);
+  const data = computeBinData();
   const a = document.createElement('a');
   a.style = 'display: none';
   document.body.appendChild(a);
diff --git a/tests/test_conversions.py b/tests/test_conversions.py
index 6bfb06e..59d856d 100644
--- a/tests/test_conversions.py
+++ b/tests/test_conversions.py
@@ -129,6 +129,45 @@ SCENARIOS = [
     ),
 ]
 
+# Bytes per output value per drawMode, mirroring CONVERSION_BYTES_PER_VALUE in
+# js/script.js. Used to verify downloadBinFile()'s byte stream (via the
+# computeBinData() helper) matches the hex values shown in the text output,
+# instead of the multi-byte values (565/888) getting truncated to one byte.
+BYTES_PER_VALUE = {
+    "horizontal1bit": 1,
+    "vertical1bit": 1,
+    "horizontal565": 2,
+    "horizontal888": 4,
+    "horizontalAlpha": 1,
+}
+
+# Scenario names to also verify against computeBinData(); restricted to ones
+# using the default plain/comma-separated output (outputFormat and separator
+# untouched), since that's the format downloadBinFile's parser expects.
+BIN_CHECK_NAMES = {
+    "default",
+    "draw_mode_vertical1bit",
+    "draw_mode_horizontal565",
+    "draw_mode_horizontal888",
+    "draw_mode_horizontal_alpha",
+    "rotate_90_horizontal565",
+    "rotate_90_horizontal888",
+    "rotate_90_horizontal_alpha",
+}
+
+
+def expected_bin_bytes(output, draw_mode):
+    """Reconstruct the byte stream a correct downloadBinFile() should produce
+    from the plain-format hex text output, expanding each value back to its
+    full byte width (MSB first) instead of assuming one byte per value."""
+    bytes_per_value = BYTES_PER_VALUE[draw_mode]
+    values = (int(tok, 16) for tok in re.findall(r"0x[0-9a-fA-F]+", output))
+    result = []
+    for value in values:
+        for shift in range((bytes_per_value - 1) * 8, -1, -8):
+            result.append((value >> shift) & 0xFF)
+    return result
+
 
 def reset_canvas_size(page, width, height):
     """Set canvas size through the real width/height text inputs."""
@@ -272,6 +311,15 @@ def main():
 
             print(f"ok     {name}")
 
+            if name in BIN_CHECK_NAMES:
+                expected_bytes = expected_bin_bytes(output, settings["drawMode"])
+                actual_bytes = page.evaluate("Array.from(computeBinData())")
+                if actual_bytes != expected_bytes:
+                    failures.append(f"{name}: bin data mismatch")
+                    print(f"FAIL   {name} (bin)")
+                else:
+                    print(f"ok     {name} (bin)")
+
             if roundtrip_eligible(settings):
                 width, height = resize or (NATIVE_WIDTH, NATIVE_HEIGHT)
                 roundtripped = run_roundtrip(page, output, settings["drawMode"], width, height)