The temperature dependent crystallization of quenched syndiotactic polystyrene is characterized by in-situ positron annihilation spectroscopy. The experimental observations obtained using differential scanning calorimetry and dynamic mechanical thermal analysis are consistent with the positron results. It is shown that the changes in thermal and mechanical properties which occur during solid state crystallization are related to the changes in positron annihilation parameters which are in turn related to free volume in the specimen. (C) 2003 Kluwer Academic Publishers.