In this study, the biosorption of Basic Blue 9 (BB9) dye from aqueous solutions onto a biomass of Euphorbia rigida was examined by means of the initial biosorbate concentration, biosorbent amount, particle size, and pH. Biosorption of BB9 onto E. rigida increases with both the initial biosorbate concentration and biosorbent amount, whereas decreases with the increasing particle size. The experimental data indicated that the biosorption isotherms are well-described by the Langmuir equilibrium isotherm equation at 20, 30, and 40 degrees C. Maximum biosorption capacity was 3.28 x 10(-4) mol g(-1) at 40 degrees C. The biosorption kinetics of BB9 obeys the pseudo-second-order kinetic model. The thermodynamic parameters such as Delta G degrees, Delta H degrees and Delta S degrees were calculated to estimate the nature of biosorption. These experimental results have indicated that E. rigida has the potential to act as a biosorbent for the removal of Basic Blue 9 from aqueous solutions.