Combined use of positron annihilation lifetime spectroscopy-an experimental tool to investigate the intermolecular sub-nanometric spaces (free volume) in polymers-and of specific volume data supply unique information on the occupied volume in a semicrystalline polymer-poly(ethylene terephthalate)-, in particular, on its thermal behavior. From our data an expansion coefficient for the occupied volume alpha (o) = 2.9.10(-5) K-1 is obtained, when the dependence of ortho-positronium intensity on the temperature is taken into account. The free volume fraction at the glass transition temperature results f = 0.04, in agreement with analogous estimates in other semicrystalline polymers. (C) 2001 American Institute of Physics.