diff --git a/README.md b/README.md
index 0634da7..dfdafa0 100644
--- a/README.md
+++ b/README.md
@@ -93,6 +93,7 @@ After the build, the executables are at `build/run_pbwo4` and `build/run_samplin
 | CMake option | Default | Meaning |
 |---|---|---|
 | `GEANT4_INSTALL_DATA` | `ON` | Download Geant4 physics data tables (~2 GB) |
+| `GEANT4_USE_GDML` | `ON` | Build Geant4 with GDML support (requires Xerces-C); set `OFF` to build without Xerces-C |
 
 The superbuild caps parallelism at `min(nproc, 16)` to avoid memory exhaustion.
 
diff --git a/superbuild/CMakeLists.txt b/superbuild/CMakeLists.txt
index 4d58e14..5454a5d 100644
--- a/superbuild/CMakeLists.txt
+++ b/superbuild/CMakeLists.txt
@@ -18,6 +18,7 @@ set(G4_SRC     ${CMAKE_SOURCE_DIR}/../lib/geant4)
 set(MINI_SRC   ${CMAKE_SOURCE_DIR}/..)
 
 option(GEANT4_INSTALL_DATA "Download Geant4 physics data files (~2 GB)" ON)
+option(GEANT4_USE_GDML "Build Geant4 with GDML support (requires Xerces-C)" ON)
 
 # ── Geant4 ────────────────────────────────────────────────────────────────────
 ExternalProject_Add(geant4
@@ -28,7 +29,7 @@ ExternalProject_Add(geant4
     -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
     -DCMAKE_BUILD_TYPE=Release
     -DGEANT4_INSTALL_DATA=${GEANT4_INSTALL_DATA}
-    -DGEANT4_USE_GDML=ON
+    -DGEANT4_USE_GDML=${GEANT4_USE_GDML}
     -DGEANT4_BUILD_MULTITHREADED=OFF
     -DGEANT4_BUILD_TLS_MODEL=global-dynamic
   BUILD_COMMAND   cmake --build <BINARY_DIR> --parallel ${NPROC}