rhe broad spectrum of interactions hetween a polyester yarn and different organic solvents has been investigated with a new experimerltal methocl called dynamic shrinkage. This technique is based on the measurement of the length changes of polyester yarns immersed in a solvent at programmed rates of heating.The dynamic shrinkage curves of a polyester yarn in various solvents are usually composed of a noncquilibrium and an equilibrium shrinkage region. From the former, it is possible to estimate the activation energies for the shrinkage process. From the equilibrium shrinkages, two analytical approaches have been used to estimate the glass-transition temperatures of about 22 polyester-solvent systems. The lowering of the glass-transition temperature of polyester by organic solvents is expressed as a function of their total solubility parameter. A bimodal distribution with interaction maxima at solubitity parameter values of 10 and 12 is observed. 'I'he effects of solvent-induced crystallization on the transition temperatures of polyester-solvent systems are discussed.