AbstractThe effect of a range of chemically dissimilar solvents on the Tg of PET is dependent upon dispersion forces. A correlation is presented between the dispersion solubility parameters (δd) of the solvents and the reduction of theTgwhich they cause. Two experimentally determed values for the (δd) of PET are presented as evidence to support the claim that aromatic and cyclic solvents solvate the aromatic part of the repeat unit and aliphatic solvents solvate the aliphatic par