The adsorption of three metal ions, Cu, Cd and Zn, in a bicomponent mixture system by activated sludge was investigated. Two-metal sorption system efficiency was evaluated by replacing simple isotherm curves by 3-D sorption isotherm surfaces. In order to describe isotherm surfaces mathematically, three empirical models were used. The isotherms indicated a competitive uptake with Cu being preferentially adsorbed followed by Cd and Zn. For instance, at C-f[Cd] = 0.9 mmol/l and with 0.5 and 2 mmol/l Cu in the system. the Cd uptake was 0.17 and 0.08 mmol/g, respectively. Therefore, a 53% decrease of Cd biosorption was observed when Cu was present at the highest concentration. On the other hand, 0.9 mmol/l Cd caused a reduction of 54% on the Cu uptake [q(Cu) = 0.28 mmol/g] in comparison with the results obtained when Cd was present at 0.225 mmol/l [q(Cu) = 0.13 mmol/g]. The interference of Zn with the Cu and Cd uptake was much less pronounced and was observed at much higher ratios of Zn:Cu and Zn:Cd concentrations. 83% of the total metal uptake was due to Cu when the residual concentrations of Cu and Zn were the same (e.g., either 0.5 or 1 mmol/l each one). On the other hand, when the residual concentrations of Cd and Zn were the same (e.g., either 0.5 or 1.5 mmol/l each one), about 71% of the total metal uptake was due to Cd uptake. (C) 2003 Elsevier Ltd. All rights reserved.