Metal-terminated bimetallic carbide nanoparticles (NPs) of tungsten and tantalum are synthesized in a monodisperse particle size distribution of 2-3 nm. The bimetallic particles feature enhanced electrocatalytic behavior with respect to the monometallic composition. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements indicate that the Ta0.3W0.7C NPs consist of a well-mixed random alloy featuring a compressed lattice that favorably impacts stability and catalytic activity. Electrochemical testing shows that the incorporation of 30% tantalum into the tungsten carbide lattice increases the electrochemical oxidation resistance of the NPs. The onset of surface passivation in 0.5 M H2SO4 shifted from +0.2 V vs RHE to +0.45 V vs RHE, and the maximum surface oxidation current shifted from +0.4 to +0.75 V vs RHE. The activity toward hydrogen evolution (HER) of the carbon-supported Ta0.3W0.7C NPs is preserved relative to the activity of unmodified carbon-supported WC NPs. The increase in electrochemical oxidation resistance is attributed to the presence of surface Ta moieties as determined by X-ray photoelectron spectroscopy (XPS) while the preservation of the HER activity is attributed to the observed lattice compression.11924 1369113699Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy [DE-FG02-12ER16352]National Science Foundation [1122374]Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)