In this study, a carbon supported oxynitrides catalyst with ultra-low Pt concentration (2 wt%, Pt-ON/C) is synthesized from Co(NO3)(2)center dot 6H(2)O, (NH4)(6)Mo7O24 center dot 4H(2)O and PtCl4 precursors by using NH3 as reducing agent and nitrogen source. It is investigated as oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells and compared to conventional 2 wt% Pt/C (ETEK) catalyst. Electrochemical and physical properties of both materials were characterized in detail. Pt-ON/C shows competitive ORR activity similar to Pt/C (ETEK) while demonstrating an improved stability. By using post mortem analysis with transmission electron microscope/scanning transmission electron microscope (TEM/STEM), the degradation mechanisms of both catalysts are investigated. Two different dominant mechanisms were suggested to explain the decreased activity of Pt/C (ETEK) under different operation protocols: for an accelerated stress test (AST) with low maximum potentials, a loss of Pt surface area associated with carbon oxidation leads to the decreased activity; while for AST with high maximum potentials, Pt particle growth, detachment, dissolution/re-deposition and severe carbon corrosion dominate the performance loss. In addition, in the catalyst of Pt-ON/C, Mo dissolution occurs under the entire potential window which, however, leads to the enhanced activity after lifetime stability test protocol.