Cobalt tetramethoxyphenyl porphyrin (CoTMPP) adsorbed on a high area carbon support (Vulcan XC72-R) and heat-treated at 900 degrees C under inert atmosphere was studied as electrocatalyst for the reduction of O-2 to H2O2 in acid medium. Experiments performed on rotating ring-disc electrode (RRDE) and gas diffusion electrode (GDE) show that the catalyst performance depends on the cobalt loading, going through a maximum at 0.2 wt. % Co. For higher cobalt loadings, a growing part of oxygen is reduced into water, decreasing therefore the selectivity of the catalyst. These results are interpreted in terms of a further reduction of H2O2 on Co-based catalytic sites before leaving the catalytic layer. For a GDE polarized at -150 mV vs. saturated calomel electrode (SCE) and loaded with 0.9 mu g cm(-2) of 0.2 wt. % Co-based catalyst, a H2O2 production rate of 300 mu mol h(-1) cm(-2) was obtained which is five times higher than the H2O2 production rate measured with Vulcan. In these conditions, the selectivity of the Co-based catalyst for H2O2 production is 92%. The good agreement observed between RRDE and GDE results confirms the relevance of using RRDE experiment for screening these non-precious metal catalysts for further GDE applications.