The adsorption of molecular oxygen at defective edge sites of zigzag and armchair graphite surfaces has been investigated by adopting cluster models in conjunction with density functional theory. Several different types of chemisorbed O-2 species are identified. It was found that the defect edge sites exhibit the significant catalytic role toward the adsorption and activation of molecular oxygen. The O-2 molecule is not only able to strongly bind to these edge sites, but the O-O bond strength is obviously weakened. Moreover, the calculated adsorption energy for O-2 adsorbed on the clean graphite basal surface is fairly consistent with the weak interaction nature Of O-2 with the surface observed in the experiment, indicating one-layer cluster model is an effective way to study O-2 adsorption on graphite surface in terms of accuracy and computational cost, which is in agreement with previous experience. Whereas, we note that the local detailed arrangement of edge carbon atoms can play an important effect on the adsorption Of O-2 on defect surfaces. (C) 2004 Elsevier B.V. All rights reserved.