Syndiotactic polystyrene (sPS) is a semicrystalline polymer for which several crystalline structures can be obtained. Cold crystallization of an amorphous sPS sample results in the formation of the α-phase, for which a close packing of the chains is hindered by the phenyl rings. This results in the unusual situation that the density of the crystalline phase (1.03 g cm -3) is lower than that of amorphous sPS (1.04 g cm-3). It is shown that as a result a line where the difference in specific volume between the liquid and crystal is zero (ΔV = 0 line) is observed in the p-T phase diagram (similar to P4MP1). At the point where this line intersects with the melting line the slope of the melting line becomes negative. Another remarkable phenomenon is that the crystalline α-phase disorders either with the application of pressure at room temperature (below Tg) or on cooling isobarically at high pressure. These observations, obtained via WAXD, SAXS, HP-DSC, and Raman spectroscopy experiments, give an indication for the existence of a reentrant phase behavior, first described by Tamman in 1903. © 2005 American Chemical Society.