Positron annihilation lifetime spectroscopy (PALS) was used t o investigate the phase transitions, mainly the glass transition, of poly(ethy1ene terephthalate) (PET) and nylon-6 during the thermal treatment of these polymers. T h e longest-lived component lifetime and intensity, indicative of ortho-positronium pick-off, exhibit thermal dependencies that can be attributed to the free-volume changes associated with structural transitions. Glass transition temperatures and the volume of intermolecular-space holes among polymer chains were obtained from the lifetime, T ~, and intensity of formation, Z3, of the long-lived component of ortho-positronium. For P E T , the free-volume fraction and thermal expansion coefficients related to the free-volume fraction were also obtained. Double glass transition behavior was noted in the analyzed polymers, which was consistent with their semicrystalline nature as revealed by differential scanning calorimetry. Increases in the slope of the lifetimetemperature plots for nylon-6 and PET were interpreted t o suggest that glass transitions are followed by a n increased free-volume cavity expansion as temperature is increased. T h e intensity response for PET was consistent with t h e association of glass transition with t h e reduction of crystalline constraint on segmental mobility in t h e amorphous phase. In contrast, t h e intensity behavior during t h e thermal treatment of nylon-6 seems t o be governed more by t h e electronic effects occurring when t h e polymer chains acquire mobility t h a n by free-volume changes. Since t h e sensitivity of P A L S is in t h e order of nanometers, it is expected t o give a n alternative novel technique to estimate phase transitions a n d relaxations in polymers from t h e point of view of t h e free volume.