We have studied the adsorption and reaction of oxygen and CO on a stepped Pt surface with varying amounts of Au, using temperature-programmed desorption and reaction (TPD and TPR), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy, and steady-state reaction measurements. When the surface is fully covered with Au it is inert to oxygen adsorption and to CO oxidation, and supports only a single weakly bound CO adsorption state. The surface covered with 0.7 ML Au, however, exhibits properties different from either bare Pt or bare Au. Our TPD and LEED results suggest the coexistence of completely Au-covered regions and regions with Au on the step edges but not on the terraces. Dissociative oxygen adsorption is reduced by 90%, and the remaining oxygen is confined to Pt sites near the Au/Pt boundaries. The Au-covered regions support weakly bound CO adsorption states with desorption temperatures of 120, 190, and 240 K. CO in these states can diffuse rapidly and react efficiently with adsorbed atomic oxygen at temperatures as low as 150 K. In low-temperature TPR experiments the reaction is limited by the availability of adsorbed oxygen under almost all conditions. Under steady-state conditions, however, it is limited by the availability of CO even at low temperatures and CO partial pressures up to 10(-6) Torr. Adding CO partial pressure does not inhibit the reaction. Consequently, adsorbed CO does not completely block all the sites at which oxygen dissociates on this surface, unlike on bare platinum.