Thin-film libraries of CoxC1-x-yNy (0 < x < 0.107, 0.003 < y < 0.389) were prepared by combinatorial magnetron sputter deposition in an Ar/N-2 gas mixture followed by subsequent heat-treatment at 700, 800, or 1000 degrees C in N-2 atmosphere. By increasing the nitrogen partial pressure during sputtering, the nitrogen content was increased significantly in the as-sputtered libraries and more nitrogen remained in the libraries after heat-treatment. The catalytic activities of the libraries towards the oxygen reduction reaction (ORR) were studied using the rotating ring-disk electrode (RRDE) technique. CoxC1-x-yNy libraries heat-treated at 800 degrees C with 0.004 < x < 0.105 and 0.026 < y < 0.097 showed good catalytic activities towards ORR in 0.1 M HClO4 solution at room temperature. The heating temperature that induces the onset of catalytic activity coincides with the temperature at which both substantial nitrogen release from the originally amorphous films and the formation of graphitic carbon and beta-Co occurs. This temperature varies significantly with both the Co and N content in the films. The production of H2O2 and the corrosion stability of the libraries are also discussed.