The sorption of Cd(II) and Pb(II) from aqueous solutions by a low-cost sorbent (leonardite) was investigated. The effect of pH, contact time and initial metal concentration was studied. The sorption of the two metals was pH dependant, the optimum range being 5-6. Batch kinetic studies showed that equilibrium was reached after 2 It. The sorption data were correlated with the Langmuir and Freundlich adsorption models. The maximum adsorption capacities obtained from the Langmuir isotherms were 0.45 mmol g(-1) (50.6 mg g(-1)) and 1.21 mmol g(-1)(250.7 mg g(-1)) for Cd(II) and Pb(II), respectively. An extensive study of the competitive adsorption between both metals was performed under the same conditions of the individual experiments. The extended Langmuir model was used to fit the experimental data to a series of 3D response surfaces. For binary systems, the maximum adsorption capacities of Cd(II) and Pb(II) were calculated as 0.27 mmol g(-1) (30.6 mg g(-1)) and 0.57 mmol g(-1) (118.7 mg g(-1)), respectively. Cd(II) adsorption was found to be more sensitive than Pb(II) to the presence of a competitive sorbate. (C) 2005 Published by Elsevier B.V.