In spite of a great deal of measurements concerning the relative free volume in polymers, the 'occupied volume' remained, up to the present, a concept not subjected to experimental investigations. However, the various theories on the free volume more or less explicitly refer to different, possible definitions of the occupied volume, in particular to its thermal behaviour. In the present work it is shown that it is possible to get information on the occupied volume by coupling positron annihilation lifetime spectroscopy and specific volume measurements; furthermore, some experimental data are discussed and compared to theoretical predictions.