A ternary composition spread, (Pt1-xRux)(1-y)Mo-y, 0 < x < 1; 0 < y < 0.3, was prepared through sputter deposition onto a nanostructured thin film support. The film was found to be reasonably stable when exposed to acid at 80 degrees C, although there was evidence for loss of some Mo, presumably through a corrosion mechanism. The catalytic activity towards hydrogen oxidation of this composition range was measured simultaneously in a 64-electrode proton exchange membrane fuel cell with emphasis on performance in the presence of CO. The addition of either Mo or Ru to Pt led to a reduction in hydrogen oxidation overpotential for a simulated reformate gas stream containing up to 50 ppm CO. The best performance under CO-containing reformate was found for compositions containing both Ru and Mo, e. g., Pt0.40Ru0.35Mo0.25. The performance observed was significantly better than that measured on compositions containing Pt and Ru only. The use of air bleed was found to be most beneficial for compositions containing predominantly Pt and Ru. (c) 2007 The Electrochemical Society.