The adsorption of heavy metal ions (Cu, Cd, Co and Pb) onto oxidized synthetic activated carbon SP800-Ox modified with phosphoric acid has been investigated. It has been shown that metal adsorption order depends on solution pH and metal ion concentration. Metal ion adsorption was modeled by the surface complexation model (DDL) where carbon is represented as four independent monoprotic acid sites (super-acidic, phosphorus-containing, carboxylic and phenolic). Initial estimation of the surface group density and acidity were made on the base of single ion binding data (proton binding) analysis by a continuous distribution approach. Calculated metal ion binding constants suggest formation of only monodentate charged complex with composition ( dropSOMe(+)), where dropSO(-) is deprotonated phosphoric, carboxylic or phenolic group. No other surface complexes were found to be significant. Super-acidic group did not participate in metal ion binding at all. (C) 2003 Elsevier B.V. All rights reserved.