Du Pont E-120 high modulus pitch-based carbon fiber was potentiostatically oxidized in (NH4)2CO3solution and analyzed with both core level and valence band XPS. Compared to the untreated E-120 fiber, this electrochemically oxidized fiber had a much higher oxygen content on its surface. Nitrogen was also found on this carbon fiber sample after the treatment in (NH4)2CO3solution, but no nitrogen was found either before or after the treatment in HNO3solution. Our previously reported work [see 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 shows that there were three different types of oxygen-containing functional groups formed on the fiber surface when the fiber was electrochemically oxidized in (NH4)2CO3solution, but only one type of oxygen-containing functionality formed after the electrochemical treatment in HNO3solution. We also reported [see Y. Xie and P. M. A. Sherwood, Appl. Spectrosc.46, 1158 (1991)] that 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 very similar.