the main purpose of the chemical modification of poly/ethylene terephthalate/ is to match the properties of the polyester fibers to the natural ones and in particular to produce the pet fibers of improved dyebility. in the reported study, the synthesis of anionically modified poly/ethylene terephthalate/ has been carried out using the monopotassium salt of di-et sulfoterephthalate as a modifying compound. the incorporation into the macromolecules of the poly/ethylene terephthalate/ of the repeat units containing potassium salt of sulfoterephthalic acid causes changes not only in the chemical composition of the polyester, but also in the supermolecular structure of the formed fiber. the structural changes make the diffusion of disperse dyes easier. moreover, the presence of chemically active groups makes it possible to dye the polyester with ionic dyes. the purpose of the reported investigations was to compare the structural and sorptive properties of the fibers made of the modified and nonmodified polyester. data are presented which confirm that the supermolecular structure of the fiber made of the modified pet differs from that made of pure polyester in contents of noncrystalline regions and in their density and orientation. these differences are probably caused by different crystallization rate of copolyester and also by different flexibilities of the polymer chains. 2 refs