diff --git a/xnat_tools/bids_postprocess.py b/xnat_tools/bids_postprocess.py
index d82f1484..6f77d3cd 100644
--- a/xnat_tools/bids_postprocess.py
+++ b/xnat_tools/bids_postprocess.py
@@ -5,6 +5,7 @@
 import typer
 
 from xnat_tools.bids_utils import (
+    append_phase_units_field,
     insert_intended_for_fmap,
     path_string_preprocess,
     remove_func_acquisition_duration_field,
@@ -109,6 +110,13 @@ def bids_postprocess(
             includesess,
         )
 
+        append_phase_units_field(
+            bids_experiment_dir,
+            includesubj,
+            session_suffix,
+            includesess,
+        )
+
     else:
         if includesubj == []:
             files = os.listdir(bids_experiment_dir)
@@ -161,6 +169,13 @@ def bids_postprocess(
             includesess,
         )
 
+        append_phase_units_field(
+            bids_experiment_dir,
+            includesubj,
+            session,
+            includesess,
+        )
+
 
 def main():
     """Entry point for console_scripts"""
diff --git a/xnat_tools/bids_utils.py b/xnat_tools/bids_utils.py
index 86123a21..19166dad 100644
--- a/xnat_tools/bids_utils.py
+++ b/xnat_tools/bids_utils.py
@@ -101,8 +101,6 @@ def build_sessions_list(bids_dir, subj, session="", sess_list=None):
     subj_path = f"{bids_dir}/sub-{subj}"
     subj_sub_dirs = os.listdir(subj_path)
 
-    log_info(f"Processing participant {subj} at path {subj_path}")
-
     # If a session is provided, only process that session.
     # If the includesess list is not empty, cocatenate all session
     # suffixes with "ses-" prefix for the file path.
@@ -117,6 +115,51 @@ def build_sessions_list(bids_dir, subj, session="", sess_list=None):
     return sessions
 
 
+def ensure_json_field(json_path, field, default):
+    """Add 'Units' key with default value if missing from JSON."""
+    try:
+        with open(json_path, "r", encoding="utf-8") as f:
+            data = json.load(f)
+
+        if field not in data:
+            data[field] = default
+
+            with open(json_path, "w", encoding="utf-8") as f:
+                json.dump(data, f, indent=4)
+            log_info(f"Added {field} field to {json_path}")
+        else:
+            log_info(f"{field} field already present in {json_path}")
+    except Exception as e:
+        log_info(f"Error processing {json_path}: {e}")
+
+
+def append_phase_units_field(bids_dir, sub_list=None, session="", sess_list=None):
+    for subj in sub_list:
+
+        # makes list of the sessions to process
+        sessions = build_sessions_list(bids_dir, subj, session, sess_list)
+
+        for sess in sessions:
+            ses_dir = os.path.join(bids_dir, f"sub-{subj}", f"{sess}")
+            if not os.path.isdir(ses_dir):
+                continue
+
+            log_info(f"Checking for missing phase units in jsons at path {ses_dir}")
+
+            # Walk through all folders under the session directory (anat, func, etc.)
+            for root, _, files in os.walk(ses_dir):
+                json_files = [
+                    os.path.join(root, f)
+                    for f in files
+                    if f.endswith("json") and "_part-phase" in f
+                ]
+
+                for path in json_files:
+                    # Phase images must have units "rad" or "arbitrary"
+                    # (defaulting to "arbitrary" here)
+                    ensure_json_field(path, "Units", "arbitrary")
+
+
 def remove_func_acquisition_duration_field(bids_dir, sub_list=None, session="", sess_list=None):
     """Remove AcquisitionDuration from func jsons if RepetitionTime is defined"""
 
@@ -128,12 +171,15 @@ def remove_func_acquisition_duration_field(bids_dir, sub_list=None, session="",
         _logger.info(f"List of sessions sub-directories {sessions}")
 
         for sess in sessions:
+
             func_path = f"{bids_dir}/sub-{subj}/{sess}/func"
 
             # Don't do anything if this session doesn't contain a func folder
             if not os.path.exists(func_path):
                 continue
 
+            log_info(f"Removing AcquisitionDuration field from jsons at path {func_path}")
+
             func_jsons = [
                 os.path.join(func_path, f) for f in os.listdir(func_path) if f.endswith("json")
             ]
@@ -170,6 +216,7 @@ def correct_for_bids_schema_validator(bids_dir, sub_list=None, session="", sess_
         sub_list = [x.removeprefix("sub-") for x in os.listdir(bids_dir) if x.startswith("sub-")]
 
     remove_func_acquisition_duration_field(bids_dir, sub_list, session, sess_list)
+    append_phase_units_field(bids_dir, sub_list, session, sess_list)
 
 
 # Extract aquisition token from filename