The performance of hydrogen fuel cells based on the crystalline solid proton conductor CsH2PO4 is circumscribed by the mass activity of platinum oxygen reduction catalysts in the cathode. Here we report on the first application of an alloy catalyst in a solid acid fuel cell, and demonstrate a mass activity 4.5 times greater than Pt at 0.8 V. This activity enhancement was obtained with platinum-palladium alloys that were vapor-deposited directly on CsH2PO4 at 210 degrees C. Catalyst mass activity peaks at a composition of 84 at% Pd, though smaller activity enhancements are observed for catalyst compositions exceeding 50 at% Pd. Prior to fuel cell testing, Pd-rich catalysts display lattice parameter expansions of up to 2% due to the presence of interstitial carbon. After fuel cell testing, a Pt-Pd solid solution absent of lattice dilatation and depleted in carbon is recovered. The structural evolution of the catalysts is correlated with catalyst de-activation. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.002303jes] All rights reserved.