In this study, transition metal complexes of amines and macrocycle ligands like cyclam, porphyrin, and phthalocyanine were studied as catalysts with Pt for electrochemical oxidation of hydrogen containing CO. These complexes are believed to act as redox mediators in generating species that oxidize CO. A systematic approach was used to improve CO tolerance of Pt electrocatalysts using the redox potential of the macrocycle complex as a guideline. Anode polarization for oxidation of H-2 containing 104 ppm CO was lowered when ruthenium tetramethylcyclam (RuTMC), molybdenum tetrakis(methoxyphenyl) porphyrin (MoTMPP), and hexaammine ruthenium chloride were used as co-catalysts with Pt. The anode polarization is reduced by 40% when MoTMPP is used as co-catalyst with Pt. The anode polarization for these catalysts shows a direct correlation with the redox potential of the complexes and decreases linearly with a reduction in the redox potential of the complex. Further improvements in CO tolerance could be obtained by increasing the electroactive population of the complex and its proximity to Pt crystallites. (C) 2004 The Electrochemical Society.