The equilibrium uptake of phenol and nickel(If) ions, both singly and in combination, by dried aerobic activated sludge was studied in a batch system. From the previous studies, the optimum biosorption pH values were determined as 4.5 for nickel(II) and as 1.0 for phenol. Adsorption isotherms were developed for both the single- and dual-component systems at these two pH values and expressed by the mono- and multi-component Langmuir, Freundlich and Redlich-Peterson adsorption models and model parameters were estimated by the non-linear regression method. It was seen that the mono-component adsorption equilibrium data fitted very well to the Redlich-Peterson model for both the components and for both the pH values while all the multi-component adsorption models adequately predicted the multi-component adsorption equilibrium data at moderate ranges of concentration. (C) 2000 Elsevier Science B.V. All rights reserved.