Zirconium-based compounds, such as ZrOxNy, have been investigated as a cathode of polymer electrolyte fuel cells. ZrOxNy was prepared using radio-frequency (rf) magnetron sputtering under an Ar + O-2 + N-2 with heating the substrate. The solubility of ZrOxNy deposited at 500 degrees C was less than 5 X 10(-8) mol dm(-3) in 0.1 mol dm(-3) H2SO4 at 30 degrees C under atmospheric condition, indicating that ZrOxNy had a high chemical stability in acidic media. The effect of the heat-treatment during the film deposition on the catalytic activity for oxygen reduction reaction (ORR) and the properties of ZrOxNy has been examined. The ORR current density on ZrOxNy deposited at a substrate temperature of 800 degrees C was similar to 30 times greater than that at 50 degrees C. The dependence of both the ORR current density at 0.4 V and the apparent activation energy of the ORR at 0.4 V on the substrate temperature changed near 500 degrees C. The crystalline structure of ZrOxNy also changed near 500 degrees C. In addition, the catalytic activity for the ORR increased with the increasing crystallinity and decreasing ionization potential of the specimens. Therefore, the crystalline structure, the crystallinity, and the ionization potential might affect the catalytic activity for the ORR. (c) 2007 The Electrochemical Society.