The effects of thermal treatments on the phase organization in some polymorphic forms of syndiotactic polystyrene have been investigated. In particular the thermal stability of the delta form was studied using x-ray diffraction and infrared spectroscopy as analytical techniques. The obtained results show that this form, characterized by chains in helix conformation, is affected by non-equilibrium phenomena. Moreover, the transition from the delta to the gamma form occurs through an intermediate form which maintains the helix chain conformation but partially loses the crystalline order.