Five catalysts (Au/MnOx, Pt/Vulcan-XC-72, Pt/SnO2/Vulcan-XC-72, Ir/carbon, and Ir/CoOx-Al2O3/carbon) were investigated as CO selective oxidation catalysts with O-2 under conditions similar to that of an operating fuel cell (76 degreesC, high humidity). This investigation was done to study the possibility of integrating a CO removal reactor into a fuel cell stack. The Ir/CoOx-Al2O3/carbon catalyst was the superior catalyst for CO selective oxidation with the highest CO conversion and selectivity among the catalysts evaluated. A nearly 100% CO conversion occurred with the Ir/CoOx-Al2O3/carbon catalyst at O-2/CO = 1.5. Although the Au/MnOx catalyst had high activity and selectivity for CO oxidation tinder dry conditions, its activity decayed dramatically due to instability of MnOx in the presence of water vapor and hydrogen.