Fe-N-C and Co-N-C materials are promising catalysts for reducing oxygen in fuel cells. The degradation of such catalysts induced by H2O2 was investigated by contacting them ex situ with various amounts of H2O2. The degradation increased with increasing amounts of H2O2. The effect was most severe for Cr-N-C followed by Fe-N-C and last by Co-N-C. Treatment with H2O2 leads to diminished oxygen reduction activity at high potential and/or reduced transport properties at high current density in fuel cell. From spectroscopic characterization, it was found that 66 and 80% of the CoNxCy and FeNxCy moieties present in pristine catalysts survived the extensive H2O2 treatment, respectively. In parallel, the activity for oxygen reduction was divided by ca 6-10 for Fe-N-C and by ca 3 for Co-N-C. The results suggest that the main degradation mechanism in fuel cell for such catalysts is due to a chemical reaction with H2O2 that is generated during operation. The super-proportional decrease of the oxygen reduction activity with loss of FeNxCy and CoNxCy moieties suggests either that only a small fraction of such moieties are initially located on the top surface, or that their turnover frequency for oxygen reduction was drastically reduced due to surface oxidation by H2O2. (C) The Author(s) 2015. Published by ECS. All rights reserved.