Electrochemical methanol oxidation was performed using p-Si(100)/Pt, p-Si(111)/Pt, and graphite foil/Pt electrodes in 0.1 M H2SO4 electrolyte at 80 degrees C. For p-Si(100) substrates, the real-area 10 min current density peaked at 1.1 mA cm(r)(-2) at a Pt thickness of 110 Angstrom at 0.1 M and 1.7 mA cm(-2) at 0.5 M methanol. The peak current was observed near 100 Angstrom for all voltages from 0.05 to 0.45 V vs Ag/AgCl. Structural and area effects were ruled out as the cause for the peak. No current/thickness peak was observed with the graphite foil substrate. A surface state model for the main reaction intermediate and poison combined with hot holes accounted for the peak in the steady-state current density and the dependence of the measured activation energy on Pt thickness. A kinetic model was used to determine rate constants as well as transient and time evolution of the poison coverage. Steady-state poison coverage ranged from 0.8 for a Pt foil to a 0.35 minimum for 110 Angstrom Pt on p-Si(100).