Applying a synthesis procedure that promotes alloy formation, highly-dispersed alumina-supported Pt-Pd catalysts spanning the Pt-rich half of the composition range were prepared, aged under oxygen-rich hydrothermal conditions, and characterized by CO chemisorption, XRD, and TEM. Anomalously large particles, typical of pure Pt catalysts treated under such conditions, were not found in any of the Pd-containing catalysts, and the extent of particle coarsening due to aging was found to decrease with increasing Pd content. Alloying appears to have little effect on the NO oxidation turn-over frequency, which increases with decreasing dispersion.