The polarization behavior of a Pt microdisk electrode, a platinized Pt microdisk electrode and a Pt-black powder microelectrode immersed in sulfuric acid solution, in contact with a moist Nafion(R) membrane were studied by cyclic voltammetry. The experimental results reveal that the surface diffusion of the adsorbed species plays an important role in determining the kinetics of interfacial reactions taking place at the powder/Nafion(R) membrane interface. The weakly adsorbed hydrogen atoms diffuse significantly faster than the strongly adsorbed hydrogen and the oxygen-containing surface species. Surface active sites in the range of tens of microns from the powder/Nafion(R) interface can be involved in interfacial reactions. The apparent coefficients of diffusion of various surface species were estimated from the variation of peak current with scan rate of potential. The apparently higher irreversibility of electrode reaction observed at the powder/membrane composite electrode is explained by considering mechanism of current generation of this type of electrode.