The variations of the structure and the activity of a sputtered Co-C-N sample for the oxygen reduction reaction (ORR) with heat-treatment temperature are studied. The sample begins to show activity for ORR in acidic electrolyte at a critical heat-treatment temperature (T-c) of 725 degrees C, at which both substantial nitrogen release from the originally amorphous films and the transformation to a heterogeneous structure (beta-Co + N-containing graphitic carbon) occurs. The sample shows activity in alkaline electrolyte when heated above 500 degrees C, the activity increases substantially near T-c and the highest activity occurs at 850 C. This suggests that active sites, apparently created below T-c, which are active below T-c, are blocked in acidic electrolyte. When N loss, graphitization and formation of beta-Co occurs at T-c, sites apparently become available in acidic electrolyte. The variation of the transformation temperature, T-c, with x and y in sputtered CoxC1-x-yNy combinatorial libraries was mapped using electron microprobe and X-ray diffraction studies. T-c decreases strongly as the Co content increases since Co is a graphitization catalyst and T-c increases weakly as the N content increases. (C) 2007 The Electrochemical Society.