The biosorption of Pb(II) and Cu(II) ions, both individually and in mixtures, by Rhizopus arrhizus was investigated in a batch system as a function of single- and dual-metal ion concentrations. The studied initial pH value for single Pb(II) and Cu(II) biosorptions was 5. It was observed that the co-ion effect on the equilibrium uptake became more noticeable as the co-ion concentration in solution increased. Adsorption isotherms were developed for both the single- and dual-metal ion systems at this pH value and expressed by the mono- and multi-component Langmuir and Freundlich adsorption models. Model parameters were estimated by non-linear regression using MAT-LAB and EXCEL softwares. It was observed that the mono-component adsorption equilibrium data fitted adequately to both the monocomponent adsorption models, for both the components and the pH value studied, while the multi-component Freundlich adsorption model predicted the multi-component adsorption equilibrium data for the ranges of initial mixture concentrations studied, and for the pH value used. (c) 2005 Elsevier Ltd. All rights reserved.