Biosorption of each of the heavy metals, Zn(II) and Cu(II), and of the binary mixture of these two metal ions by the indigenous Thiobacillus thiooxidans was investigated in this study. Equilibrium concentration (q(m)) and dissociation constant (K-d) were calculated by fitting the experimental data with the Langmuir isotherms. The effects of pH, pretreatment of biomass, and temperature on the amount of metal uptake by this organism were also determined. Typically, the adsorption capacity increases with increasing pH in the ranges of 2.0-6.0 and 4.0-5.0 for Zn(II) and Cu(II), respectively. Chemical pretreatment of the biomass with 0.075 M NaOH has positive effects on its capacity for metal biosorption. Higher temperature yields higher biosorption capacity for both metals. The indigenous T thiooxidans is in favor of Zn(II) uptake in the binary mixture. Biosorption of Cu(II) is inhibited by the existence of Zn(II). The total amount of metal adsorbed in the binary mixture decreases in comparison with biosorption of only one kind of metal ion. (C) 2003 Elsevier B.V. All rights reserved.