The biosorption characteristics of Pb(II) and Cd(II) ions from aqueous solution using the green alga (Ulva lactuca) biomass were investigated as a function of pH, biomass dosage, contact time, and temperature. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models were applied to describe the biosorption isotherm of the metal ions by U. lactuca biomass. Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The monolayer biosorption capacity of U. lactuca biomass for Pb(II) and Cd(II) ions was found to be 34.7mg/g and 29.2mg/g, respectively. From the D-R isotherm model, the mean free energy was calculated as 10.4kJ/mol for Pb(II) biosorption and 9.6kJ/mol for Cd(II) biosorption, indicating that the biosorption of both metal ions was taken place by chemisorption. The calculated thermodynamic parameters (DeltaG degrees , DeltaH degrees and DeltaS degrees ) showed that the biosorption of Pb(II) and Cd(II) ions onto U. lactuca biomass was feasible, spontaneous and exothermic under examined conditions. Experimental data were also tested in terms of biosorption kinetics using pseudo-first-order and pseudo-second-order kinetic models. The results showed that the biosorption processes of both metal ions followed well pseudo-second-order kinetics.