The stability of Pt-based or other metallic alloys under the corrosive working conditions of proton exchange membrane fuel cells (PEMFCs) is an important factor to consider in the search for improved PEM electrocatalysts. The dissolution of transition metals (Co, Ni, Mn, Fe) from Pt1-x-yMxM'(y) electrocatalysts after operation in PEM hydrogen fuel cells or after treatment in 1 M H2SO4 at 80 degrees C is reported. Catalyst libraries were prepared by combinational sputtering on nanostructured thin-film supports that were used directly in fuel cells and for acid testing. In each library the Pt mass loading was kept fixed at 0.1 mg cm(-2) while the amounts of M and M' varied with position. The transition metal content of all compositions of all libraries, determined by electron microprobe, was significantly reduced in the same way after fuel cell or acid testing. Samples with x + y >= 0.25 before fuel cell or acid testing were found with x + y approximate to 0.25 after exposure due to the dissolution of transition metals, independent of the choice of M and M' for the elements studied here. We suggest that the composition Pt1-x-yMxM'(y) with x + y approximate to 0.25 is near the percolation limit for diffusion of transition metals from the interior of the alloy. The acid treatment described here mimics fuel cell testing from a corrosion standpoint and hence can be used as a simpler, ex situ test for PEM electrocatalysts. (c) 2006 The Electrochemical Society.