Conversion of energy stored in renewable fuels for sustained and environment friendly operation necessitates new technologies. Polymer electrolyte membrane fuel cells represent an energy conversion technology that has advantages of high operating efficiencies with low hazardous emissions. However the discovery of more active and poison tolerant catalysts for anode and cathode reactions remains a major barrier to commercialization of this technology. The search for improved catalyst formulations is hindered by the massive parameter space available for their construction. Combinatorial methods represent an exploration process well suited to accelerate this discovery through the ability to generate and screen a multitude of compositions in a single experiment. In this work, we describe a combinatorial strategy to generate a wide variety of catalyst compositions (single/binary) and interrogate their activity directly in an electrochemical environment. Catalyst library fabrication tools based upon array deposition methods were used to prepare samples possessing catalyst combinations containing Pt and additional metals such as Ru, Rh, Mo, Ir, Nb, Ta, Pd, Sn, Os and W. High throughput screening of catalytic activity was accomplished by scanning electrochemical microscopy (SECM). Reactivity maps were constructed by directly measuring the kinetics of hydrogen oxidation with a scanning microelectrode probe in the presence and absence of adsorbed carbon monoxide. Results provide quantitative rate constants for hydrogen oxidation and poison tolerance over a broad sampling of catalyst compositions.