Oxidation of CO and methanol on a polycrystalline platinum electrode in 0.1 M HClO4 was examined using real time optical second harmonic generation (SHG). A large SHG signal was observed in the potential range where CO directly adsorbed or formed from dissociative chemisorption of methanol on the electrode surface. The potential dependence of the SHG intensity for the surface covered with CO at saturation was interpreted to be due to the redistribution of the pi-electrons in CO rather than the changes in the free electron density on the metal side. A simplified kinetic model for the SHG intensity measured as a function of time at a given electrode potential step allowed to evaluate the rate of CO buildup on the surface from methanol decomposition. It was also found that weakly adsorbed hydrogen blocks effectively methanol from adsorbing on to the surface by eliminating the atop sites.