The kinetics of the electrochem. oxidn. of a CO adlayer on Pt[n(111)*(111)] electrodes in 0.5M H2SO4 was studied using chronoamperometry. The objective is to elucidate the effect of the cryst. defects on the rate of the reaction by using stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approxn. for the Langmuir-Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Pt[n(111)*(111)] surfaces under electrochem. conditions. The apparent rate const. for the electrochem. CO oxidn., detd. by a fitting of the exptl. data with the mean-field model, is proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidn. An apparent intrinsic rate const. is detd. The potential dependence of the apparent rate consts. is structure insensitive with a Tafel slope of .apprx.80 mV/dec, suggesting the presence of a slow chem. step in an ECE reaction mechanism.