The dissolution of Fe and Ni from Pt1-xMx( M = Fe, Ni; 0 < x < 1) oxygen reduction electrocatalysts was studied under simulated operating conditions (low pH, 80 degreesC) of proton exchange membrane (PEM) fuel cells. The alloys were prepared combinatorially by sputtering Pt and M (M = Fe, Ni) onto thin films of nanostructured whisker-like supports, and mapped over the entire composition range of the binary systems. For 0 < x < 1.0, we observe the formation of randomly ordered substitutional solid solutions of Pt1-xFex and Pt1-xNix alloys. Electron microprobe measurements show that transition metals are removed from all compositions during acid treatment, but that the percentage removed increases with x, acid strength, and temperature. For small values of x (x < 0.6) no substantial changes in the lattice size are observed upon dissolution of Fe or Ni suggesting that the dissolved transition metals originate from the surface. However, for electrocatalysts with x> 0.6, the lattice constant expands indicating that transition metals dissolve also from the bulk. X-ray photoelectron spectroscopy results show complete removal of surface Ni (Fe) after acid treatment at 80 degreesC for all compositions. The results of the acid treatments compare well to the composition changes that occur when a Pt1-xFex or Pt1-xNix combinatorial catalyst library is used in an operating PEM fuel cell. (C) 2004 The Electrochemical Society.