The oxygen reduction activity of several Pt1-xCox (0 <= x <= 0.5) thin-film libraries, sputter deposited on glassy carbon disks, were studied in 0.1 M HClO4 using a rotating ring-disk electrode (RRDE) setup. All as-deposited compositions were face centered cubic structure. Further crystallization and grain growth was observed upon annealing. Bulk and surface compositions of the electrocatalysts were measured before and after RRDE measurements using electron microprobe and X-ray photoelectron spectroscopy. Surface Co loss, by dissolution, was observed for all samples. The as-deposited samples did not show a kinetic improvement over Pt; however, after annealing Pt1-xCox, a 1.7-fold gain in the area-specific activity was observed for 0.2 <= x <= 0.5. (c) 2007 The Electrochemical Society.