High energy ball milling, an industrially amenable technique, has been used to produce CO tolerant unsupported Pt-Ru based catalysts for the oxidation of hydrogen in polymer electrolyte fuel cells. Nanocrystalline Pt-0.5-Ru-0.5 alloys are easily obtained by ball-milling but their performances as anode catalysts are poor because nanocrystals composing the material aggregate during milling into larger particles. The result is a low specific area material. Improved specific areas were obtained by milling together Pt, Ru and a metal leacheable after the milling step. The best results were obtained by milling Pt, Ru, and Al in a 1:1:8 atomic ratio. After leaching Al, this catalyst (Pt-0.5-Ru-0.5 (Al-4)) displays a specific area of 38 m(2)g(1)(-). Pt-0.5-Ru-0.5 (Al-4) is a composite catalyst. It consists of two components: (i) small crystallites (similar to 4 nm) of a Pt-Al solid solution (1-3 Al wt%) of low Ru content, and (ii) larger Ru crystallites. It shows hydrogen oxidation performance and CO tolerance equivalent to those of Pt-0.5-Ru-0.5 Black from Johnson Matthey, the commercial catalyst which was found to be the most CO tolerant one in this study.