Abstract: On the assumption that displacement is controlled by surface reaction and equilibrium of H2 and D2 in gas phase is able to be established quickly, one dimensional model characterizing displacement between protium and deuterium in LaNi5 and LaNi4.7 Al0.3 hydride was applied to simulate procedure of the displacement. In the model, isotopic exchange probability is variable and other parameters come from refer ence or experiment. Composition of gas exiting in LaNi5 and LaNi4.7Al0.3 bed was calculated and compared with experimental data. Results show that effluent curves agree with experimental data while isotopic exchange probability is 2. 96×107. The model can be used to describe displacement between hydrogen isotopes in metal hydride.