The biosorption equilibrium isotherms of Cu(II) and Ni(II) from wastewater onto marine brown algae Cystoseira indica, which was chemically modified by various methods were studied and modeled. To predict the biosorption isotherms and to determine the characteristic parameters for process design, 21 one-, two-, three-, four-, and five-parameter isotherm models were applied to experimental data. The interaction among biosorbed molecules is repulsive and there is no association between them, and 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 (0.9899-0.9976), low standard error (0.1202-0.0144), and sum of square error (0.0144-0.0495) 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) > Langmuir (two-parameter) > Radke-Prausnitz-II (three-parameter) > Fritz-Schluender (four-parameter) > Temkin (two-parameter). (C) 2008 American Institute of Chemical Engineers AIChE J, 54: 3291-3302, 2008