Functional groups on the surface of Polyacrylonitrile (PAN)-based carbon fibers and in fiber surface sizing are likely to react during the curing process of composites, and these reactions could affect the infiltration and adhesion between the carbon fibers and resin. T300B-3000-40B fibers and fiber surface sizing were heat-treated at different temperatures, and the structural changes of both the fiber surface sizing and extracted sizing after heat treatment were investigated by Fourier transform infrared spectroscopy. The results show that the concentration of epoxy groups in both the fiber surface sizing and extracted sizing decreased with increasing heat-treatment temperature and decreased to zero after treatment at 200 degrees C. The concentration of epoxy groups in the extracted sizing was lower than that of the fiber surface sizing after treatment under the same conditions; this indicated that the rate of reaction between the carbon fibers and fiber surface sizing was higher than the reaction rate of the fiber surface sizing system. X-ray photoelectron spectroscopy analysis reveals that the content of C-O bonds and activated carbon atoms on the surface of the desized treated carbon fibers was the highest when the heat-treatment temperature was 150 degrees C; this proved the reaction between the carbon fibers and the fiber surface sizing. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012