Platinum alloy catalysts (Pt0.5Co0.3Cr0.2) were supported on carbon to achieve 10, 15, and 20% platinum. The oxygen-reduction performances of these supported catalysts were evaluated for phosphoric acid fuel-cell cathodes. The higher mass fractions of catalyst allow for thinner catalyst layers. Theoretical modeling predicts unproved performance under some conditions. The platinum alloy catalysts were fabricated into electrodes, and improved performance was seen experimentally in a half-cell apparatus.