The H-D isotope effect on the electrode kinetics of a metal-proton conductor interface has been investigated. The current-voltage behaviour depends on the nature of the electrode (Ni, Ag), the atmosphere (H(2), D(2)), the partial pressures of the gases, and the temperature. The isotope effect was manifested by a reduction in anodic current densities when H(2) was replaced by D(2). For both nickel and silver, the anodic part of the polarisation curves displayed limiting current behaviour, due to concentration polarisation. The empirical reaction order was not influenced by isotope replacement, suggesting that the mechanism of reaction is decisively determined by the electrode. Under the specific condition of 1% H(2) and 1% D(2) in the atmosphere and 800 degrees C, the electrochemical isotope effect, given by the limiting current, was 1.4+/-0.15 in the case of Ni and 1.2+/-0.3 in the case of Ag. The empirical activation energy of the limiting current was affected by both the isotope used and the nature of the electrode. For both metals, a higher activation energy was observed with deuterium than with hydrogen. (C) 1999 Elsevier Science B.V. All rights reserved.