diff --git a/NPLib/Physics/NPNucleus.h b/NPLib/Physics/NPNucleus.h
index e3e12992e250e9a80e0b2e9d91a9ac0b5e77d85b..b983cc8faf1eb10fbc408c415b58855195b709f6 100644
--- a/NPLib/Physics/NPNucleus.h
+++ b/NPLib/Physics/NPNucleus.h
@@ -120,7 +120,7 @@ namespace NPL {
     void				SetBeta(double beta)					{fBeta = beta; BetaToGamma(); BetaToEnergy(); EnergyToTof(); EnergyToBrho();BetaToVelocity();}
     
     // Nuclear mass in MeV
-    double      Mass() const {return (fAtomicWeight*amu_c2 + fMassExcess/1000.);}
+    double      Mass() const {return (fAtomicWeight*amu_c2 + fMassExcess/1000. - fCharge*electron_mass_c2);}
     void        Print() const   ;
   };
 }