We report an application of the scanning electrochemical microscope that exploits its ability to spatially map the kinetics of heterogeneous electron-transfer reactions in order to perform screening measurements for combinatorial studies of electrooxidation catalysts. The ability to measure the activity of catalyst surfaces toward the hydrogen oxidation reaction via tip−sample feedback is used to characterize the activity of PtxRuy and PtxRuyMoz catalysts as a function of composition and electrode potential. Multielement band electrodes containing various compositions of PtxRuy and PtxRuyMoz deposits are created via pulsed electrochemical deposition onto patterned substrates. Catalyst compositions are verified through a combination of Auger electron spectroscopy and energy-dispersive X-ray spectroscopy. Activity toward the hydrogen oxidation reaction is probed in sulfuric acid solutions by using a scanning microelectrode tip placed in close proximity to the catalyst surfaces. The tip potential is held at a value where protons are reduced to hydrogen at a diffusion-limited rate. Tip-produced hydrogen is converted back to protons via oxidation at the catalyst surfaces. This leads to an increase in feedback current at the tip, whose magnitude directly reflects the substrate's rate constant for hydrogen oxidation. Monitoring the feedback response while scanning the microelectrode tip over catalyst samples of various compositions is used to deduce the onset of activity. The onset of hydrogen oxidation on these PtxRuy and PtxRuyMoz samples in the presence of an adsorbed monolayer of carbon monoxide is determined by performing screening studies as a function of electrode potential. The compositions with the lowest onset potentials are identified, and the results are compared with carbon monoxide stripping experiments.