High strength carbon fibers were electrochemically treated in an aqueous ammonium carbonate solution, with increasing electric current density, using an original continuous treatment process. The electrochemical treatment induced a modification of the surface properties, i.e., surface functional groups as observed by XPS. As a result, the O1s/C1s and the N1s/C1s ratios of the fiber surfaces increased with the increase of current intensity of the electrochemical treatment in the interval of about 30-50 A m-2. However, no significant change in the surface functional activity characters was observed for strong treatments. Hence, a moderate treatment (30 A m-2) was sufficient to obtain optimum (O1s + N1s)/C1s ratios in this system. This treatment is possibly suitable for carbon fibers to be incorporated in a polar organic matrix, resulting in increasing the interlaminar shear strength (ILSS) of the resulting composites. Copyright 1999 Academic Press.