In the presence of methanol and dissolved oxygen, hydrogen adsorption and reduction behaviours on electrodeposited Pt electrode were investigated with cyclic voltammetry and in situ electrochemical quartz crystal microbalance. Specific hydrogen adsorptions occur at two different cathodic potentials and its adsorption at low cathodic potential is suppressed by oxygen adsorption. After methanol oxidation occurred on Pt electrode, part of Pt surface is covered with reaction intermediate, CO, which blocks the adsorption of hydrogen and oxygen. Measurement of reaction currents and mass changes during cyclic voltammetry and potentiostatic oxygen reduction suggested that the amount of hydrogen adsorption decreases significantly and specific hydrogen adsorption steps are altered, which resulted in the decrease of oxygen reduction. (C) 2004 Elsevier Ltd. All rights reserved.