The effect of isotopic substitution of H by D on the electrochemical cycling properties as well as on the electrochemical and solid-gas isotherms of LaNi4.5Mn0.5 intermetallic compound has been investigated. Contrary to what is observed in the solid-gas isotherms, significant isotope effects have been found in the electrochemical isotherms. This is due to different H2O-D2O electrolyte activities and H2O/H-2-D2O/D-2 standard electrode potentials. An empirical method has been developed for converting electrode potentials into equivalent H-2 and D-2 pressures based on the Nernst equation. Hydrogen and deuterium equilibrium pressures obtained by this method agree with those obtained by solid-gas reactions. Isotope effects in the rate of H/D transfer across the electrode surface have also been observed. The activation period of the electrode is much longer in D2O electrolyte. This effect is ascribed to slower permeation rates of D atoms as compared to H atoms through the native oxide barriers at the electrode surface. Moreover, higher charge-transfer overpotential has been observed for D than for H atoms. (c) 2007 The Electrochemical Society.