CO oxidation reaction was studied over monometallic and bimetallic palladium-platinum catalysts at 25:75, 50:50, and 75:25 Pd:Pt atomic fractions co-impregnated over a gamma-Al2O3 support. The size of the metal particles, measured by modified hydrogen chemisorption, increased as the Pd fractions in the catalyst increased. The surface compositions and site metal distributions of the catalysts determined from Monte Carlo simulations indicated that Pd atoms segregated to the surface: at low Pd levels, Pd occupied preferentially 6 and 7 coordinated defect-like sites. As the Pd fraction increased, first 8 coordinated (100) and then 9 coordinated (111) planes were populated. At low temperatures (below 423 K), CO oxidation reaction over bi-metallic catalysts behaved like the monometallic Pd catalyst suggesting that Pd atoms segregated to the surface. The results of Monte Carlo studies and reaction tests together were interpreted as the governing steps in CO oxidation primarily taking place at the defect-like sites.