Catalytic CO oxidation was investigated over Pt, Rh, and Pt-Rh bimetallic catalysts supported on gamma-Al2O3. Both dry and wet calcination procedures were performed to control Cl content in the final products. The catalysts were characterized with hydrogen chemisorption and analytical electron microscopy (AEM). A steady-state plug-flow reactor was used for activity measurement. An automated on-line gas chromatograph was employed to analyze CO, CO2, and O-2. Based upon our previous findings, we were able to prepare catalysts with only Pt-rich or only Rh-rich particles. The Cl concentration in these catalysts did not affect activity significantly. In net-oxidizing conditions, platinum was found to be more active than rhodium. The presence of synergism depends upon a certain metal particle composition-size distribution of Pt-rich alloy particles. The current investigation has also demonstrated that feed composition and operating temperature have a strong influence on the presence of the synergistic effect, The possible mechanisms of the observed synergism are discussed.