The equilibrium rotational constants of an isotopically substituted series of molecules are supposed to be unaffected by molecular isotopic substitution in the Born-Oppenheimer approximation. Hydrogen provides a unique example for the determination of an isotopically independent B e value (known as the effective B e value) since there is a nine-fold increase in mass from 1 H 2 to 3 H 2 . Using the most precise values of B e obtained experimentally from vibration-rotational spectroscopy, the effective B e value of hydrogen has been evaluated as 60.846 ± 0.006 cm - 1 , from which an isotopically independent r e value of 74.129 ± 0.008 pm has been obtained. Comparison is made with the experimentally derived, uncorrected r e values of the hydrogen isotopic species 1 2 , 1 H 2 H, 2 H 2 , 1 H 3 H, 2 H 3 H and 3 H 2 from rotational spectroscopy.