Polyacrylonitrile copolymer fibers have been heated in air to different maximum heat treatment temperatures (MHTT) from 190 to 270degreesC. The functionalities of C, O, and N in the precursors were determined by X-ray photoelectron spectroscopy. The intensity of the C1s spectra line 286.8ev, which is identified as C-O and C = N functionalities, was found to initially increase and suddenly decrease above 230degreesC. Additionally, the significant broadening of the N1s spectra line was also observed at the same MHTT. It is suggested that some oxygen was incorporated into the six-member heterocyclic structures that formed by cyclization of nitrile groups and transformed into pyrrole-type structures by the elimination of carbon monoxide. (C) 2004 Wiley Periodicals, Inc.