|Biomass of non-living brown seaweed Sargassum fluitans pretreated with NaOH is capable of taking up more than 10% (q_max: 3.85 mmol/g for Al and 1.48 mmol/g for Cu) of its dry weight in the Al and Cu at pH of 4.5. However, the maximum Al and Cu uptakes calculated from Langmuir isotherm were 1.58 mmol/g for Al and 1.35 mmol/g for Cu at pH 3.5. Equilibrium batch sorption study was performed using two-metal system containing Al and Cu. The mathematical model of the two-metal sorption system enabled quantitative estimation of one-metal biosorption inhibition due to the influence of a second metal. NaOH-treated S. fluitans contained 2.19 mmol (43 wt.%) carboxyl groups per gram of biomass. A modified form of Langmuir, which assumes binding of Cu as Cu^(2+) and Al as Al(OH)_(2)^(+), was used to model the experimental data. This result agrees with the one of mono-valent sorption for Al in single-metal system. The modified Langmuir model gives the following affinity correlated coefficients: 0.196 for Cu and 6.820 for Al at pH 4.5, and 2.904 for Cu and 3.131 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 give Cu and Al uptakes reduced by 82.5% and 5.4%, respectively. However, these values at pH 3.5 were 55% (Cu) and 31% (Al).