Highly dispersed PtCo-CeOx couples of fine Pt3Co alloy particles with adjacent CeOx are synthesized on a carbon support using the modified colloid method. The average sizes Of Pt3Co and CeOx particles are approximately 1.7 and 4 nm, respectively. According to XPS studies, the CeOx, particle consists of a CeO2 phase crystal for the core and a Ce2O3 phase for the shell. The CeOx-containing electrocatalysts of PtCo-CeOx and Pt-CeOx show improved electrochemical performances for the oxygen reduction reaction compared with NCO and Pt electrocatalysts without CeOx, in both ring-disc electrode and cell tests. It seems that the unsaturated Ce2O3 phase of the CeOx. surface causes the facile adsorption and desorption of oxygen, leading to an enhanced oxygen supply to the PtCo alloy. (C) 2008 Elsevier B.V. All rights reserved.