Hercules AU4 PAN-based carbon fiber surface was analyzed with both core level and valence band XPS. Compared to Du Pont and Amoco's pitch-based carbon fibers with different modulus, the AU4 fiber had more oxidized components on the surface than the pitch-based carbon fibers. [See Y. Xie and P. M. A. Sherwood, Appl. Spectrosc.43, 1153 (1989); Chem. Mater.1, 417 (1989);2, 293 (1990); Appl. Spectrosc.44, 797 (1990); Chem. Mater.3, 164 (1991); Appl. Spectrosc.44, 1621 (1990);45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. A. Sherwood,ibid.46, 645 (1992).] The AU4 PAN-based fiber had a much less graphitic structure in both the surface and the bulk than the pitch-based fibers as evidenced by both XPS and XRD. It also had a small amount of nitrogen in its structure, but no nitrogen was found on the pitch-based carbon fiber surfaces. Our previously reported work [Y. Xie and P. M. A. Sherwood, Chem. Mater.1, 427 (1989);2, 293 (1990); Appl. Spectrosc.44, 797 (1990); Chem. Mater.3, 164 (1991); Appl. Spectrosc.44, 1621 (1990);45, 1158 (1991); Y. Xie, T. Wang, O. Franklin, and P. M. A. Sherwood,ibid.46, 645 (1992)], showed that the XPS valence band spectra were more sensitive to chemical environment on the carbon fiber surface than core level spectra and could be well interpreted by X–αcalculations with model compounds. In this work, the valence band spectrum showed that there were nitrogen and at least two different types of oxygen species on the AU4 fiber surface.