diff --git a/generate_endf.py b/generate_endf.py
index dbc6133..0941d6b 100755
--- a/generate_endf.py
+++ b/generate_endf.py
@@ -16,7 +16,7 @@
 from shutil import rmtree, copy, copyfileobj
 
 import openmc.data
-from utils import download, process_neutron, process_thermal
+from utils import download, process_neutron, process_thermal, update_zsymam
 
 
 class CustomFormatter(argparse.ArgumentDefaultsHelpFormatter,
@@ -459,6 +459,11 @@ def sort_key(path):
 
     if 'thermal' in args.particles:
         particle = 'thermal'
+        if args.release == 'viii.1':
+            # Patch non-unique ZSYMAM fields
+            update_zsymam(thermal_dir / 'tsl-UinUO2-5P.endf',   'UUO2-5P')
+            update_zsymam(thermal_dir / 'tsl-UinUO2-10P.endf',  'UUO2-10P')
+            update_zsymam(thermal_dir / 'tsl-UinUO2-100P.endf', 'UUO2-100P')
         with Pool() as pool:
             details = release_details[args.release][particle]
             results = []