Abstract: The surface modification of multiwall carbon nanotubes (MWCTNs) was achieved by electrochemical anodic oxidation in NaOH and H₂SO₄ electrolytes. Their defect structures, functional groups, morphology and dispersibility in aqueous solutions were characterized by SEM, TEM, XPS, FTIR, Raman spectroscopy, zeta potential analysis and a stability test of their suspensions. Results indicate that anodic oxidation with the NaOH electrolyte removes more amorphous carbon, introduces fewer defects and more oxygen-containing functional groups (mainly -OH), and produces shorter nanotubes, as a result of which the resulting nanotubes are more stable when dispersed in aqueous solutions compared with those oxidized by the H₂SO₄ electrolyte. MWCTNs are more easily oxidized in the NaOH electrolyte, and the numbers of oxygen-containing functional groups and hydroxyl groups increase continuously with the oxidation degree. In the H₂SO₄ electrolyte, however, the numbers of oxygen-containing functional groups and hydroxyl groups increase and level off with the oxidation degree.