Sargassum muticum biomass was chemically modified to test its sorption capacity toward a model organic cation: methylene blue. Two different classes of treatments, specifically the esterification of the carboxylic acids and the extraction of the lipid fraction, have been applied. Chemical modification of the biomass increases the sorption capacity, especially if the lipid fraction is removed from the algae. The maximum dye uptake was obtained for biomass obtained after the extraction of the lipid fraction by means of acetone under reflux treatment, with a q(e) value of 860 mg.g(-1) from the Langmuir isotherm. Maximum uptakes were found in the pH range of 4 to 10. The equilibrium was achieved in (30 to 60) min, depending on the algal pretreatment. The pseudo-first-order empirical model can describe the process as a whole. Plots of the sorption capacity q(t) versus the square root of time, at the initial stages of the sorption process, fit the intraparticle diffusion equation, so an intraparticle diffusion coefficient of 5.46.10(-8) cm(2).s(-1) is obtained for methylene blue in chemically modified S. muticum. Specific surface areas of the involved biomaterials are calculated from maximum uptakes at equilibrium and critically analyzed.