The present study deals with the competitive adsorption of cadmium (Cd(II)) and nickel (Ni(II)) ions from aqueous solution on to rice husk ash (RHA). Non-competitive Redlich-Peterson (R-P) and Freundlich models represent the single metal ion equilibrium sorption data. The adsorption capacities for the binary mixtures-RHA system are in the order Ni(II) > Cd(II). The combined equilibrium sorption of Cd(II) and Ni(II) ions onto RHA is found to be antagonistic in nature and the extended Freundlich model was found to best represent the binary equilibrium isotherm data. (c) 2008 Elsevier B.V. All rights reserved.