diff --git a/llmeter/endpoints/bedrock.py b/llmeter/endpoints/bedrock.py
index 04012ca..400c995 100644
--- a/llmeter/endpoints/bedrock.py
+++ b/llmeter/endpoints/bedrock.py
@@ -1,9 +1,12 @@
 # Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 # SPDX-License-Identifier: Apache-2.0
 
+from base64 import b64decode
+from copy import deepcopy
 import logging
 import time
 from uuid import uuid4
+import warnings
 
 import boto3
 from botocore.config import Config
@@ -55,7 +58,7 @@ def __init__(
             )
         self._inference_config = inference_config
 
-    def _parse_payload(self, payload):
+    def _parse_payload(self, payload) -> str:
         """
         Parse the payload to extract text content.
 
@@ -102,6 +105,48 @@ def _parse_payload(self, payload):
             logger.error(f"Unexpected error parsing payload: {e}")
             return ""
 
+    @staticmethod
+    def _patch_base64_bytes(payload: dict) -> dict:
+        """Copy a request payload, undoing base64-encoding on image/multimedia bytes where present
+
+        Bedrock's boto3 SDK base64-encodes inline multimedia bytes automatically, and doesn't try
+        to detect whether it's already been applied - so needs raw bytes input. However, LLMeter
+        prefers to keep this data encoded so that a load test's payloads are 1/ serializable to
+        JSON and 2/ consistent with what actually gets sent 'on the wire' to the endpoint.
+
+        Therefore this method is called during invoke (before the performance counter starts), to
+        prepare the payload for boto3.
+        """
+        payload = deepcopy(payload)
+        messages = payload.get("messages", [])
+        raw_media_types_found = set()
+        for msg in messages:
+            content = msg.get("content", [])
+            for c in content:
+                for media_type in ("image", "video"):
+                    if media_type in c:
+                        image_source = c[media_type].get("source", {})
+                        if "bytes" in image_source:
+                            if isinstance(image_source["bytes"], str):
+                                try:
+                                    # Undo base64-encoding:
+                                    image_source["bytes"] = b64decode(
+                                        image_source["bytes"]
+                                    )
+                                except Exception:
+                                    raw_media_types_found.add(media_type)
+                            else:
+                                raw_media_types_found.add(media_type)
+        if raw_media_types_found:
+            warnings.warn(
+                "Bedrock payload had raw source.bytes in %s content. Although this is "
+                'supported by boto3, you probably want to `b64encode(img_data).decode("utf-8")` '
+                "them in LLMeter, so that your payloads and load test results can be saved to "
+                "(JSON) files." % (sorted(list(raw_media_types_found))),
+                RuntimeWarning,
+            )
+        return payload
+
     @staticmethod
     def create_payload(user_message: str | list[str], max_tokens: int = 256, **kwargs):
         """
@@ -233,9 +278,12 @@ def invoke(self, payload: dict, **kwargs) -> InvocationResponse:
                 payload["inferenceConfig"] = self._inference_config or {}
 
             payload["modelId"] = self.model_id
+            # Separate copy to store final modifications that *shouldn't* be visible in LLMeter
+            # results (as it's not JSON-serializable):
+            payload_patch = self._patch_base64_bytes(payload)
             try:
                 start_t = time.perf_counter()
-                client_response = self._bedrock_client.converse(**payload)  # type: ignore
+                client_response = self._bedrock_client.converse(**payload_patch)  # type: ignore
                 time_to_last_token = time.perf_counter() - start_t
             except ClientError as e:
                 logger.error(f"Bedrock API error: {e}")
@@ -268,9 +316,12 @@ def invoke(self, payload: dict, **kwargs) -> InvocationResponse:
             payload["inferenceConfig"] = self._inference_config or {}
 
         payload["modelId"] = self.model_id
-        start_t = time.perf_counter()
+        # Separate copy to store final modifications that *shouldn't* be visible in LLMeter
+        # results (as it's not JSON-serializable):
+        payload_patch = self._patch_base64_bytes(payload)
         try:
-            client_response = self._bedrock_client.converse_stream(**payload)  # type: ignore
+            start_t = time.perf_counter()
+            client_response = self._bedrock_client.converse_stream(**payload_patch)  # type: ignore
         except (ClientError, Exception) as e:
             logger.error(e)
             return InvocationResponse.error_output(