The reaction sites of CO oxidation induced by 193 nm irradiation were studied in the coadlayer of CO and O 2 on Pt(113) = (s)2(111) (001) around 110 K with the angular and velocity distributions of desorbing product CO 2 . At low coverages, the CO 2 desorption takes place in surface troughs. On the other hand, three kinds of reaction sites are found at high O 2 coverage, where three additional desorption components, peaking along the (111) terrace, the (001) step and the bulk surface normal, become prodominant.