Biomass of non-living brown seaweed Sargassum fluitans pretreated by different methods is capable of taking up more than 10% (11 meq/g) of its dry weight in aluminum at a pH of 4.5. It is indicated that the biomass sequestered the aluminum in the form of polynuclear aluminum species. The fraction of Al(OH)_(3) precipitated in the aluminum nitrate solution without biomass at pH 4.5 increased as the Al concentration increased. Aluminum-alginate complex precipitated in the solution as alginate was partially released from the biomass. External colloidal precipitate occurring in native and protonated S. fluitans biomass sorption systems caused a significant difference in Al sorption isotherms determined by standard and desorption methods, respectively. Sodium ions added for pH adjustment were not sorbed at all in the presence of aluminum ions.