X-ray photoelectron spectroscopy (XPS) has been used to investigate the surface characteristics of two carbon fibers and determine the role of surface treatment in improving the properties of a carbon fiber composite. Carbon and oxygen were the major elements observed on the surface of the carbon fiber samples. Small amounts of lightly oxidized nitrogen amounting to about 3 at.% were detected on the surface of thermally oxidized carbon fiber in a gas mixture of O-2/(N-2 + O-2). Graphitic carbon was the major carbon functional component on the surface of carbon fiber samples but other functional forms of carbon were also present such as C-O, C=O, and O-C=O. The biggest differences in the surface chemistry between the carbon fibers with and without surface treatment was in the carbonyl group (C=O) content. Thermal oxidation processing in a gas mixture of O-2/(N-2 + O-2) can be employed to increase the carbonyl functional group which is the main contributor in improving the interlaminar shear strengths of carbon fiber composite materials. (C) 2001 Elsevier Science B.V. All rights reserved.