In this study, the biosorption of copper(II) ions on Enteromorpha prolifera, a green seaweed. was investigated in a batch system. The effects of operating parameters such as initial pH, temperature, initial metal ion concentration and biosorbent concentration on the copper(II) biosorption were analysed using response surface methodology (RSM). The proposed quadratic model for central composite design (CCD) fitted very well to the experimental data that it could be used to navigate the design space according to ANOVA results. The optimum biosorption conditions were determined as initial pH 4.0, temperature 25 degrees C, biosorbent concentration 1.2 g/L and initial copper(II) ion concentration 200 mg/L. The Langmuir and Freundlich isotherm models were applied to the equilibrium data at different temperatures and initial pH values. The maximum monolayer coverage capacity of E. prolifera for copper(II) ions was found to be 57.14 mg/g at 25 degrees C and initial pH 4.0 indicating that the optimum biosorption temperature and initial pH. The external and intraparticle diffusion models were also applied to biosorption data of copper(II) ions with E. prolifera and it was found that both the external diffusion as well as intraparticle diffusion contribute to the actual biosorption process. The pseudo-second order kinetic model described the copper(II) biosorption process with a good fitting. The thermodynamics of copper(II) biosorption on E. prolifera indicated its spontaneous and exothermic nature. (C) 2008 Elsevier B.V. All rights reserved.