Isotherms of hydrogen underpotential deposition on Pt(1 0 0), (1 1 0) and (1 1 1) in HClO4 and H2SO4 were obtained from cyclic voltammograms. They were used to determine Gibbs energies of adsorption, standard Gibbs energies of adsorption, interaction parameters and their derivatives, and the reference state surface coverages. The results obtained permitted us to determine the attractive/repulsive interactions existing during hydrogen adsorption on monocrystalline platinum. Simple Frumkin type isotherm may well describe H adsorption on Pt(1 0 0) in HClO4, Pt(1 1 0) in H2SO4 and Pt(1 1 1) in both acids. (C) 2003 Elsevier B.V. All rights reserved.