Biosorption of phenol by the marine seaweeds Lessonia nigrescens Bory and Macrocystis integrifolia Bory was investigated as a function of initial solution pH, showing a higher adsorption percentage at pH 10 with values between 10% and 35%, respectively. The apparent ionization constants of the algae were determined by means of the Katchalsky's theory, obtaining values close to 3.0 for both seaweeds. From the results, a strong adsorption dependence on pH is proposed at the level of the phenol's aqueous chemistry driven purely by a polar mechanism that involves the formation of hydrogen bonds with the hydroxyl groups that are spatially arranged in the polysaccharic chains that form the seaweed structure. This study shows that both marine algae are potential biosorbents in their application for the removal of phenol and derivatives from residual waters. © 2007 Elsevier B.V. All rights reserved.