Du Pont E-120 high modulus pitch based carbon fiber was potentiostatically oxidized in HNO3solution and analyzed with both core level and valence band XPS. Compared to the untreated E-120 fiber, the potentiostatically oxidized fiber had a much higher oxygen content on its surface. Both surface and bulk graphitic structures were severely disordered by the electrochemical treatment as evidenced by both XPS and XRD studies. [See Y. Xie and P. M. A. Sherwood, Appl. Spectrosc.43, 1153 (1989); 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).] However, no nitrogen was found on this pitch-based carbon fiber either before or after the treatment in HNO3solution. 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 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, together with the O 1score level spectrum, showed that there was only one single-type oxygen-containing functional group formed on the fiber surface when the fiber was electrochemically oxidized in HNO3solution. We also reported [see Y. Xie and P. M. A. Sherwood, Appl. Spectrosc.44, 1621 (1990)] that the highly oxidized carbon fiber surface functionality would gradually decompose after exposure to the x rays inside the XPS spectrometer, and that [see Y. Xie and P. M. A. Sherwood, Appl. Spectrosc.45, 1158 (1991)] the reproducibility of the potentiostatic treatment was not as good as that of the galvanostatic treatment, although the results from potentiostatic oxidation and galvanostatic oxidation were about the same.