The response of a thermoplastic polyimide (TPI) to immersion in a range of solvents was investigated by a number of experimental techniques. The kinetics of solvent diffusion were determined by weight uptake and NMR imaging, the effect of absorbed solvent on thermal behaviour was investigated by dynamic mechanical analysis and differential scanning calorimetry and the morphology of TPI exposed to solvent was determined by wide-angle X-ray scattering. The tensile properties of amorphous and crystalline TPI were measured before and after exposure to solvent. Amorphous TPI absorbed significant quantities of some solvents that lowered the T-g, but none were sufficiently plasticising to induce crystallisation at room temperature. There was a direct correlation between the extent of solvent ingress and the environmental stress cracking behaviour of amorphous TPI. Crystalline TPI absorbed only small quantities of all the solvents used, but showed limited environmental stress cracking resistance. (C) 2000 Elsevier Science Ltd. All rights reserved.