The interactions between poly(ethylene terephthalate)(PET) and several binary solvent mixtures were investigated in terms of polymer/solvent distances in the space representing three dimensional solubility paramenter. The calculated distances in the space were compared with the swelling and equilibrium crystallinity of PET. Maximum interaction took place when solubility parameters of solvent mixtures approached to those of aliphatic ester and aromatic residues of PET. The swelling and equilibrium crystallinity were also closely related to enthalphy term of Flory-Huggins interaction parameter, XH. When the mixture of acrylonitrile and styrene was used for grafting onto PET, grafting percent varied with mixing ratio of the monomers, and this was explained with the solubility parameter concept.