Syndiotactic poly(p-fluoro-styrene) (s-PPFS) has been prepared with a polymerization procedure which allows reaching high average molecular masses and satisfactory yields. The polymorphic behavior of the polymer has been mainly studied by X-ray diffraction. calorimetric and infrared analyses. The main crystalline phase of s-PPFS, obtained by melt processing or cold-crystallization, exhibits trans-planar chains, is orthorhombic (a = 9.5 angstrom, b = 28.7 angstrom, c = 5.1 angstrom) and melts at nearly 320 degrees C. The X-ray analysis shows a strict analogy of this orthorhombic phase with the P phase of s-PS, also as for the occurrence of two limit ordered (beta '') and disordered (beta') modifications, which differ for the intensity of reflections characterized by h + k = 2n + 1. A metastable crystalline Phase, also exhibiting trans-planar chains, has been observed for as-polymerized samples as well as for amorphous samples crystallized by sorption of toluene or 1,4-difluoro-benzene. Mainly on the basis of solvent sorption and desorption experiments, it is suggested that this metastable phase is a co-crystalline phase with the low-molecular-mass guest molecules. (c) 2009 Elsevier Ltd. All rights reserved.