The oxygen reduction activity of a series of Pt1-xCox catalysts (with high Co content) sputter-deposited onto 3M's nano-structured thin film support was measured with Rotating Disk electrode techniques. The oxygen reduction reaction (ORR) activity was found to be much higher than that measured on pure Pt (3-4 times higher absolute activity at 1000 mV for the same Pt loading). The ORR activity was only weakly dependent on the Co content (as-sputtered). With repeated potential cycling it was, however, somewhat unstable, likely primarily as a result of poisoning of the catalyst surface with electrolyte impurities during the long term test protocol. Ex-situ x-ray diffraction measurements of the disks during testing showed loss of bulk Co from the lattice, resulting in loss of a large fraction of the initial Co content. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3595746] All rights reserved.