We investigated the effect of CoSe2/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H2O2 production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe2 nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H2O2 production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe2 loading on the disk electrode. H2O2 production also decreased with increasing catalytic site density, a phenomenon related to the CoSe2 loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe2/C nanoparticles at 22 mu g cm(-2), we determined that the transfer process involves about 3.5 electrons. (C) 2009 Elsevier Ltd. All rights reserved.