The structural changes occurring during heating in a syndiotactic polystyrene (sPS)-solvent complex were monitored in situ by X-ray diffraction. The room temperature delta form transformed into the gamma form on heating above the glass transition temperature of the sPS. The transition temperature showed a linear dependency on the amount of solvent absorbed; the higher the solvent molecules absorbed, the higher the transition temperature. However, the transition temperature does not depend on the nature of the solvent. The emptied clathrate form transformed into the gamma form at the glass transition temperature. The gamma form transformed into the alpha" form at similar to200 degreesC on heating and is independent of the delta to gamma form transition. Calorimetric studies showed an endotherm followed by an exotherm during these transitions and indicated that the transitions are first order in nature. The studies provided information on the stability of various crystalline forms of sPS.