In this study, the competitive biosorption of iron(III) and chromium(VI) to Chlorella vulgaris from a binary metal mixture was investigated in a single-staged batch reactor as a function of V-0/X-0 (volume of solution containing heavy metal mixture/quantity of biosorbent) ratio and second metal ion concentration at pH 2. The obtained results showed that the increase in biomass quantity (or the decrease of V-0/X-0 ratio) with the addition of second metal ion affected the removed quantities of iron(III) or chromium(VI). The sorption phenomenon was expressed by the competitive, multi-component Langmuir adsorption isotherm and this expression was used for calculating each residual or adsorbed metal ion concentration at equilibrium (C-eq,C-i or C-ad,C-eq,C-i) at a constant V0X0 ratio for a given combination of heavy metal ions in a single-staged batch reactor. Experimental C-eq,C-i and C-ad,C-eq,C-i values were compared to those calculated and graphically determined. (C) 2000 Elsevier Science S.A. All rights reserved.