Nanostructured thin film catalysts (NSTF) with widely varying compositions of PtxMy and PtxMyNz (M, N = Ni, Co, Zr, Hf, Fe, Mn) have been evaluated for 0 <= x, y, z < 1. The catalysts' activity for oxygen reduction (ORR) was measured in 50 cm(2) fuel cell membrane electrode assemblies. Pt1-xNix was found to be unique in showing an extraordinarily sharp peak in ORR activity as a function of the as-made composition around x = 0.69 +/- 0.02 determined gravimetrically. This composition gave a corresponding fcc lattice parameter of 3.71 Angstroms and a grain size of 7.5 nm. Both surface area and specific activity increases contribute to the increased mass activity of the resultant dealloyed films. The ORR mass activity of the Pt3Ni7 is 60% higher than for the NSTF standard Pt68Co29Mn3 alloy. Rotating disk electrode measurements of a Pt1-xNix series on NSTF coated glassy carbon disks show a similar large and sharp peak in activity. In contrast, PtCo shows a diminished but still sharply peaked mass activity in 50 cm(2) tests near x = 0.62 by electron microprobe over the 0 < x < 0.7 range examined. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3595748] All rights reserved.