Ball-milling has been used to prepare performing CO tolerant polymer electrolyte fuel cell anode catalysts that contain Pt and Ru. The catalyst precursors are obtained by milling together Pt, Ru and a dispersing agent in the atomic ratio 0.5, 0.5 and 4.0. This precursor is not easily recovered after milling because it sticks to the walls of the vial and on the grinding balls. However, the precursor is recovered as a powder when a process control agent (PCA) is added during the milling step. Various PCAs have been used. The PCA should not interfere with the electrocatalytic activity of the catalysts obtained by leaching the precursor. The best preparation of catalyst precursors are obtained by milling: (i) Pt, Ru and Al (dispersing agent) in the atomic ratio 0.5, 0.5, 4.0 + 10 wt% NaF (PCA) or (ii) Pt , Ru and MgH2 in the 0.5, 0.5, 4.0 atomic or molecular ratio. In this case, MgH2 plays at the same time the role of a dispersing agent and that of a PCA. The catalysts are obtained by leaching Al and NaF in (i) or MgH2 in (ii). The CO tolerance of these catalysts is equivalent to that of Pt0.5Ru0.5 Black from Johnson Matthey. The ball-milled catalysts have a surface area comprised between 30 and 44 m(2) g(-1). As-prepared catalysts are mainly made of metallic Pt and metallic plus oxidized Ru. After fuel cell tests, Pt is completely metallic while the oxidized Ru content decreases but does not disappear. These catalysts are composed of particles with crystallites of two different sizes: in (i) nanocrystallites (similar to4 nm) that contain essentially Pt alloyed with Al and perhaps some Ru, and larger (greater than or equal to similar to 30 nm) crystallites that contain essentially Ru; in (ii) Pt nanocrystalline particles that may contain some Ru and larger particles that contain essentially either Ru or Pt.