The aim of this work is to improve the retention capacity already noticed for the raw Posidonia towards basic dyes and notably for basic blue 41 (BB41). To improve the fixation rate of these cationic entities, we have undertaken the introduction of carboxymethylate groups on the raw Posidonia. Thus, we have obtained materials with various carboxymethylate groups contents (from 10 to 30%). We have then studied the fixation capacities of BB41 on the different materials. We have noticed that the retention capacity of modified Posidonia was improved compared to the raw one. We have studied the effect of different parameters influencing the fixation capacities such as initial dye concentration, support dose, contact time, pH, carboxymethylate groups content, and temperature. This phenomenon of retention is compared to classic models of Langmuir and Freundlich. Some thermodynamic parameters such as free energy (Delta G(0)), enthalpy (Delta H-0), and entropy (Delta S-0) changes have been studied. (c) 2006 Wiley Periodicals, Inc.