The yield of complete oxidation product (CO2) from electrocatalysis of methanol on a Pt(100) electrode in perchloric acid electrolyte at room temperature was determined by combined measurements of chronoamperometry and linear sweep voltammetry. The fractional yield of CO2 was zero over a potential range of 0.35-0.45 V-RHE and increased monotonically to unity over the potential range of 0.5-0.65 V. The results may be explained by a simple parallel mechanism, in which methanol oxidizes directly to CO2, or by a complex serial mechanism, in which the overpotential for CO oxidation is reduced by solution phase methanol.