The reaction sites involved in the CO oxidation were studied on a stepped Pt(113) [or (s)2(111) x (100)] surface through analysis of the spatial distribution of desorbing product CO2. The distribution was measured with angle-resolved thermal desorption spectroscopy, Five CO2 formation peaks are observed during heating of the coadlayer of CO and oxygen, around 370 K (P-1-CO2), 300 K (P-2-CO2), 270 K (P-3-CO2), 235 K (P-4-CO2), and 180 K (P-5-CO2). The reaction sites working: for these CO2 formations depend strongly on the CO and oxygen coverage. Generally, the CO2 formation on the (111) terrace is predominant at high CO coverage, whereas the contribution from the (001) step is enhanced at high oxygen coverage. A mechanism is proposed for this reaction site switching. (C) 1997 American Institute of Physics.