DuPont E-120 high-modulus pitch-based carbon fibers were treated electrochemically in 0.5 M (NH4)2CO3 solution under both potentiostatic and galvanostatic modes. X-ray photoelectron spectroscopy (XPS) was used to monitor the chemical changes on the carbon fiber surfaces. Both core-level and valence-band spectra showed that the treatment introduced both oxygen-containing and nitrogen-containing functional groups onto the fiber surfaces, and the mainly oxygen-containing functional groups produced were carbonyl (C=O) type functional groups after longer treatment time. For short treatment time, hydroxide (C-OH) type groups were the dominant functionality, and ether (C-O-C) or epoxide type groups were also formed. The O 2 s peaks from oxygen atoms in the hydroxide functionality and the ether or epoxide groups are well separated in the valence-band spectra; the corresponding O 1 s peaks, however, are not separated in the O 1 s core-region spectra.