Catalysts containing Pt and Mo were synthesized by high energy ballmilling. The two metal powders were milled together in one step with a leacheable dispersing agent (Al or MgH2) and a leacheable process-control agent (NaF or MgH2) to increase the specific area of the catalysts and avoid sticking of the catalyst's precursor on the vial walls and grinding balls. The catalysts were labeled Pt0.5Mo0.5(Al)(4) and Pt0.5Mo0.5(MgH2)(4) to reflect the bulk atomic or molecular nominal content of the milled powders. After the leaching step, the actual bulk Mo content of both catalysts was rather low (similar to5 atom %). Despite their low Mo content, both catalysts displayed a similar performance, in H-2 + 100 ppm CO/O-2 fuel cell tests, to that displayed by Pb0.5RU0.5 black from Johnson Matthey. The best catalyst in H-2 + 100 ppm CO was Pt0.5Mo0.5(MgH2)(4), which is a face-centered cubic solid solution of Mo (similar to5 atom %) in Pt with a specific area of 35.9 m(2)/g. By combining X-ray diffraction, X-ray photoelectron spectroscopy, and fuel cell test results, it was possible to determine that Pt was metallic in the catalyst, but that Mo(V and VI) were present at the surface of the working anode in H-2 + 100 ppm CO.