The comparative study of the interfacial properties of an anionic polysaccharide, sodium alginate (Alg), and its hydrophobically modified derivative (Alg-C-12), covalently substituted by dodecyl chains (12% mol/mol saccharide unit), was carried out in the absence or in the presence of an oppositely charged surfactant, dodecyltrimethylammonium bromide (DTAB). The drastically different behaviors which were observed are interpreted in terms of the arrangement and mobility of the hydrophobic long alkyl chains, depending on the nature of their fixation, covalent or ionic, on the polysaccharide backbone,