Carbon supported cobalt selenide (CoSe2/C) nanoparticles were prepared by a simple microwave method using cobalt acetate and selenium dioxide as precursors with different molar ratios of Se/Co. The effects of Se/Co ratios on surface morphology, crystal structure, chemical composition and electrocatalytic activity toward oxygen reduction reaction (ORR) of CoSe2/C catalyst nanoparticles were systematically investigated. It was found that the experimental compositions of CoSe1.8/C similar to CoSe2.7/C with average crystallite sizes of 15.4 similar to 12.9 nm were obtained with the Se/Co ratios of 2.0 similar to 4.0. The major phases of CoSe2/C nanoparticles were identified to be orthorhombic CoSe2 with minor cubic CoSe2. The potentials corresponding to ORR (E-ORR) reached 0.6 similar to 0.7 V, while the electron transfer numbers (n) were 3.1 similar to 4.0 in oxygen saturated sulfuric acid solutions. The formation of Se-rich CoSe2/C nanoparticles was confirmed with Se/Co >= 2.5, and the best ORR activities with E-ORR = 0.705 V and n = 4.0 could be achieved with Se/Co = 3.0. Slightly excess amount of Se oxide presented in CoSe2/C would prevent CoSe2 nanoparticles from growing and smaller sizes with less agglomerated Se-rich CoSe2/C were obtained, resulting in good ORR activity. However, too much Se oxide would cause severe aggregation of CoSe2 nanoparticles, leading to poor ORR activity. (C) 2014 Elsevier Ltd. All rights reserved.