The biosorption equilibrium isotherms of Cd(Il) and Pb(II) from aqueous solution onto both raw and treated biomass of Lobophora variegata were studied and modeled. To predict the biosorption isotherms and to determine the characteristic parameters for process design, 19 one-, two-, three-, four-, and five-parameter isotherm models were applied to experimental data. Results show that in general the accuracy of models to fit experimental data improves with the number of parameters. Biosorption isotherm modeling shows that the interaction of Cd(II) and Pb(II) with the L. variegata surface is localized to monolayer sorption. The interaction among biosorbed molecules is repulsive, and there is no association between them. Biosorption is carried out on energetically different sites and is an endothermic process. The five-parameter Fritz-Schluender model gives the most accurate fit with high R-2 (0.9960 to 0.9978), low standard error (SE) (0.0334 to 0.0129), and residual or sum of square error (SSE) (0.0066 to 0.0137) values to all experimental data in comparison to other models. The biosorption isotherm models fitted the experimental data in the order: Fritz-Schluender (five-parameter) > Toth (three-parameter) > Fritz-Schluender (four-parameter) > Langmuir (two-parameter) > Temkin (two-parameter).