The present work describes our study on the competitive biosorption of Cd(II) and Zn(II) ions onto Sargassum filipendula from single component and binary systems. Results showed that S. filipendula was an efficient biosorbent for the removal of Cd(II) and Zn(II) ions from an aqueous solution. The equilibrium experimental data for the single component system for the two metallic species fitted well to Langmuir and Langmuir-Freundlich isotherm models. Seven isotherm models were tested with the equilibrium data for modeling of the binary system. Based on the sum of squares error, the Langmuir-Freundlich isotherm model showed the best fit to the binary adsorption data. The influence of the additional cation on the behavior of the biosorption of Cd(2+) and Zn(2+) was analyzed comparing single component and binary isotherms. It was observed that Zn(2+) had a relevant effect on the Cd(2+) biosorption, but the interference of Cd(2+) on the sorption of Zn(2+) was considerably less intense. (C) 2010 Elsevier Ltd. All rights reserved.