Abstract: The dyeing behaviour of an anion-modified polyester with C. I. Basic Blue 5 in an aqueous solution of benzyl alcohol has been investigated. Sorption isotherms indicate that dye cations combine with anions within the fiber through ion exchange. It has been found by thermodynamic analysis that the dyeing process is an exothermic reaction with a decrease of entropy. The presence of benzyl alcohol in the dyeing bath raises the saturation value and the rate of dyeing, while the affinity, heat and entropy change of dyeing are lowered, thus leading to the elimination of ring-dyeing. All these effects are enhanced with the increase of the concentration of benzyl alcohol in the dyeing bath (or in the fiber).Crystallization properties of the polymer treated with an aqueous solution of benzyl alcohol have been determined by methods of infrared spectroscopy and X-ray diffraction. Drawing test as well as methods of DSC and DDV have been employed to determine the thermal and mechanical properties of this polymer. Experimental results indicate that benzyl alcohol penetrates into the polymer and weakens the intermolecular interaction in its amorphous region without any impairment of its crystalline region. Meanwhile, the fiber becomes swollen and its segmental mobility is further increased. The process of dyeing is therefore promoted.