Potentiodynamic studies of the underpotential deposition of hydrogen (H-apd) on MoPt4 electrode in 0.5 mol dm(-3) HClO4 aqueous solution in the range of temperature from 275 to 312 K are made and thermodynamic state functions for the hydrogen adsorption are determined. Theoretical treatment of the experimental results is derived from H-upd electrochemical adsorption isotherm. It is determined that DeltaG(ads)(0), (H-upd), as a function of temperature, varies with the surface coverage from ad similar to -20 kJ mol(-1) (theta = 0). The increase of DeltaG(ads)(0), (H-upd) with the surface coverage indicates the repulsive interactions between ad H-upd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and entropy of adsorption are calculated. The values of these functions are determined to be DeltaH(ads)((0=0))(0) = 4.5 kJ mol(-1) and DeltaS(ads)((0=0))(theta) = 82 J mol(-1) K-1. The value of DeltaH(ads)(0) allows determinations of the bond energy between electrode surface and Hupd, which is found to be EM-H = 213 kJ mol(-1) for theta = 0. The lateral repulsion interactions are the reasons why M-H-upd bond energy decreases significantly with the increase of coverage so the saturation coverage is much less than 1 in the UPD potential region. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.