Cu and Al sorption capacities of NaOH-treated Sargassum fluitans biomass were studied using equilibrium methodology. An evaluation of sorption performance and modeling in a two-metal system was carried out using a modified multicomponent Langmuir isotherm. The maximum Cu and Al uptakes calculated from the Langmuir isotherm were 1.54 mmol/g (9.8 weight percent) and 3.75 mmol/g (10.1 weight percent) at pH 4.5, respectively. However, these values were 1.35 mmol/g for Cu and 1.58 mmol/g for Al at pH 3.5. The modified Langmuir model gave the following affinity correlated coefficients: 0.20 for Cu and 6.82 for Al at pH 4.5, and 2.90 for Cu and 3.13 for Al at pH 3.5. The interference of Al in Cu biosorptive uptake was assessed by “cutting” the three-dimensional uptake isotherm surfaces at constant second-metal final concentrations. Equimolar final equilibrium concentrations of Cu and Al of 1 mM at pH 4.5 reduced Cu and Al uptakes by 83.4 and 5.5%, respectively. However, these reductions at pH 3.5 were 53.0 (Cu) and 29.1% (Al).