Anodes with various Pt/Ru loadings were prepared using three different anode catalysts: E-TEK 40% Pt-Ru/Vulcan XC-72 and Engelhard 40 and 50% Pt-Ru/C. These anodes were tested for their CO-tolerance at a cell temperature of 70 degreesC in the presence of 10 and 50 ppm CO, respectively. The performance declined with CO concentration for all the anodes. However, Engelhard catalysts showed better performance than E-TEK catalyst under our experimental conditions. A minimum Pt/Ru loading of 0.43, 0.22 and 0.30 mg/cm(2) was needed to achieve the maximum performance for E-TEK 40% Pt-Ru/C, Engelhard 40 and 50% Pt-Ru/C, respectively, when 10 ppm CO/70% H-2/30% CO2 was used at a cell current density of 0.20 A/cm(2). Using 50 ppm CO/70% H-2/30% CO2 and at a cell current density of 0.20 A/cm(2), a minimum Pt/Ru loading of 0.60,0.32 and 0.36 mg/cm(2) was needed to achieve the highest performance for E-TEK 40% Pt-Ru/C, Engelhard 40 and 50% Pt-Ru/C, respectively.