The competitive biosorption of cadmium(II) and nickel(II) onto dried Chlorella vulgaris, a green alga from binary metal mixture was studied and compared with single metal ion situation in a batch stirred system. The effects of single- and dual-metal ion concentrations on the biosorption capacity of biomass were investigated at an initial pH value of 4.0. Equilibrium uptake of cadmium(II) and nickel(II)) increased with increasing its initial metal ion concentration up to 150 mg 1(-1) and decreased considerably by the presence of increasing concentrations of the other metal ion. Both the Freundlich and Langmuir adsorption models were suitable for describing the biosorption equilibrium of cadmium(II) and nickel(II) ions both in single and binary systems. According to the Langmuir model the maximum biosorption capacity of dried alga was determined as 86.6 mg g(-1) for cadmium(II) and as 58.4 mg g-1 for nickel(II) for single ion situation. These values dropped to 68.5 mg g(-1) for cadmium(II) and to 28.3 mg g(-1) for nickel(II) when co-ion concentration was kept at 150 mg 1(-1). For each metal ion, variation of isotherm constants due to the level of other metal ion were expressed as a function of initial co-ion concentration by non-linear regression techniques. (c) 2005 Elsevier Ltd. All rights reserved.