A tensile rheometer capable of confining diluent is described. This rheometer makes it possible to characterize solid samples with imbibed diluent. Tensile stress-relaxation data were obtained as a function of temperature for dry glassy semicrystalline poly(ethylene terephthalate) (PET) and as a function of composition for semicrystalline PET imbibed with dimethylformamide (DMF). Data on PET / DMF captures the glass-rubber transition as DMF content is increased.