diff --git a/src/constants.f90 b/src/constants.f90
index dd26ce78c..43bb60953 100644
--- a/src/constants.f90
+++ b/src/constants.f90
@@ -9,7 +9,8 @@ module constants
   public
 
   real(pReal), parameter :: &
-    T_ROOM = 300.0_pReal, &                                                                         !< Room temperature in K
-    K_B = 1.38e-23_pReal                                                                             !< Boltzmann constant in J/Kelvin
+    T_ROOM = 300.0_pReal, &                                                                         !< Room temperature in K. ToDo: IUPAC: 298.15
+    K_B = 1.38e-23_pReal, &                                                                         !< Boltzmann constant in J/Kelvin
+    N_A = 6.02214076e-23_pReal                                                                      !< Avogadro constant in 1/mol
 
 end module constants