In order to investigate the interaction between supercritical carbon dioxide (scCO(2)) and poly(ethylene terephthalate) (PET) and the effect on this interaction of some additive modifiers such as alcohols, the swelling behaviors of PET in scCO(2)/modifier systems were examined. A special apparatus equipped with two observation windows was constructed. The change in the dimension of the PET yarn in the scCO(2)/modifier was monitored using a CCD camera pointing through one of the windows. It was confirmed that the swelling of PET due to scCO(2) is accompanied by the crystallization of PET, but the crystallization induced by scCO(2) Can be prevented by adding specific modifiers to the fluid. The equilibrium swelling of PET in scCO(2)/modifier increased as the solubility parameter of the fluid approached to the solubility parameter of PET. (c) 2005 Elsevier B.V. All rights reserved.