Mg-doped LaCoO3 particles were prepared in various particle sizes via a modified sol-gel method using excess MgO as pinning particles. The adsorbed oxygen species of the products were studied by XPS, TPD-O-2 and TPR-H-2. The amount of adsorbed oxygen varied with changes in average particle size and had an important effect on the catalytic activity towards CO oxidation. XPS and TPD-O-2 results indicated that there were two kinds of surface oxygen species and that the relative ratio of the surface adsorbed oxygen to lattice oxygen increased with an increasing BET surface area. The catalytic activities of Mg-doped LaCoO3 particles were clearly enhanced with the increase of surface area and the amount of adsorbed oxygen. (C) 2011 Elsevier B.V. All rights reserved.