Polyesters have excellent fiber properties such as chemical, heat and light stability, and high strength. However, polyesters, especially polyester fibers and fabrics, are difficult to dye. The molecular structure and the high levels of orientation and crystallinity that impart the desirable properties to the polyester also contribute to a resistance to coloration by dye compounds.Ionic groups may be introduced into the polymer chain of normal polyester, so that it becomes dyeable with cationic and acid dyes.