The simultaneous biosorption of copper(II) and chromium(VI) to C. vulgaris from binary metal mixtures was investigated in a single-staged batch reactor as a function of V-0/X-0 ratio (volume of wastewater containing heavy metal mixture/quantity of biosorbent) at different orders of second metal ion addition and at pH values of 2.0 and 4.0 chosen as the optimum biosorption pH values for chromium(VI) and copper(II), respectively. The increase in biomass quantity (or the decrease of V-0/X-0 ratio) with the addition of the second metal ion affected the removed quantities of chromium(VI) and copper(II) due to pH. The sorption phenomenon was expressed by the competitive, multi-component Freundlich 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 V-0/X-0 ratio for a given combination of heavy metals in a single-staged batch reactor. Experimental C-eq,C-i values were compared to those calculated and graphically determined.