In this work, we apply a recently developed selective site blocking with gold adatoms (Kuznetsov et al. Electrocatal 3:211–220, 2012) for studying the electrooxidation of adsorbed carbon monoxide on electrodeposited Pt. The reaction is studied using chronoamperometry, and the obtained current transients are fitted with a nucleation-and-growth model in order to determine the characteristic kinetic parameters on defect and terrace (facet) sites. We conclude that Au selectively blocks defect sites up to very high coverages of 0.42 MLs, strongly slowing down the reaction kinetics. Blocking of defect sites with Au adatoms also results in an increase of the packing density of CO on terrace sites. The rate constant of CO oxidation on defects is higher than that on terraces and exhibits higher Tafel slope.