diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4a4a325..274be3c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ This file documents all notable changes to the GEOS-Chem Classic wrapper reposit
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased] - TBD
+### Changed
+- CMake now throws an error if FAST-JX is used with any other mechanism than Hg
+
 ## [14.4.1] - 2024-06-28
 ### Fixed
 - Fixed formatting error in `.github/workflows/stale.yml` that caused the Mark Stale Issues action not to run
diff --git a/CMakeScripts/GC-ConfigureClassic.cmake b/CMakeScripts/GC-ConfigureClassic.cmake
index b8d1ca1..eb311c0 100755
--- a/CMakeScripts/GC-ConfigureClassic.cmake
+++ b/CMakeScripts/GC-ConfigureClassic.cmake
@@ -192,12 +192,18 @@ function(configureGCClassic)
     #-------------------------------------------------------------------------
     # Use Fast-JX rather than Cloud-J?
     #-------------------------------------------------------------------------
-
     set(FASTJX OFF CACHE BOOL
         "Switch to use legacy FAST-JX in GEOS-Chem"
     )
     gc_pretty_print(VARIABLE FASTJX IS_BOOLEAN)
     if(${FASTJX})
+        #---------------------------------------------------------------------
+        # Throw an error unless we are using the Hg mechanism,
+        # The fullchem & custom mechanisms now use Cloud-J!
+        if(NOT ${MECH} MATCHES "Hg")
+          message(FATAL_ERROR "FASTJX can only be used with the Hg mechanism!")
+        endif()
+        #---------------------------------------------------------------------
         target_compile_definitions(GEOSChemBuildProperties
             INTERFACE FASTJX
         )