An acid dyeable polypropylene fiber has been prepared by admiring an additive to the polymer prior to extrusion. An additive has been synthesized by attaching long alkyl chains together with substituents carrying basic nitrogen to a rigid polymer backbone. The dyeing behavior of the physically modified fiber has been characterized by kinetic and thermodynamic parameters. A diffusion coefficient of the order of 10(-11) m(2)/s was found, indicating very fast diffusion of the dye molecules within the fiber. The measured thermodynamic parameters provide an insight into molecular interactions that are of prevailing electrostatic nature. The results have been explained in terms of the binding of dye molecules to the basic dye sites supplied to the fiber by an additive. The change of dyeability of modified fiber at about 70 degrees C is attributable to the glass-rubber transition of the additive. From among the two models of diffusion considered, the pore model and the free volume model, the first one seems to be dominant.