Dissolved humic substances occur naturally in most surface waters. Such material interferes with conventional drinking water treatment processes and adsorbs on the surface of granular activated carbon (GAC), decreasing the effectiveness and life of GAC filters. An alkali treatment was investigated as a means of desorbing humic substances from the activated carbon surface and possibly increasing the bed life of the filters. A maximum of (60 %) removal was achieved using this method. Surface titrations were undertaken using activated carbon samples with adsorbed humic material before and after the alkali desorption treatment. It was found that the surface charge properties of the samples were not significantly altered by the treatment, and that the major influence on the surface properties was the concentration of the adsorbed humic material. The degree of ionization of the adsorbed material was found to be very strongly influenced by the surface concentration, and this effect was interpreted in terms of the orientation of the adsorbed molecules and the very strong electrostatic interaction between charged groups of the adsorbed material. 1994 Academic Press, Inc.