The depth profiles of protium and deuterium which were charged electrolytically, were measured by elastic recoil detection analysis (ERDA) and secondary ion mass spectrometry (SIMS) techniques in order to study the isotope effect in hydrogen absorption of Ti, Zr, Nb, Ni and Pd. The absolute loading ratios of H(D)/metal were calculated from the ERDA spectra and the depth profiles of SIMS were compared with the results of the ERDA. The isotope absorption ratios are estimated to be (D/H)(Ti) = 0.43, (D/H)(Zr) = 0.53, (D/H)(Nb) = 0.17 and (D/H)(Pd) = 0.10. The content in Ni is below the detection limit. The mass balance equations based on the transport-absorption model, were applied to analysis of the experimental results. This model reveals that the isotope absorption ratios for the Nb and Pd cases are governed mainly by the flux of hydrogen ions flowing to the surface of the metal electrode. However, the competition between the absorption-conversion process and the recombination process mainly determine the isotope ratio for the Ti and Zr Cases.