The phase behavior of syndiotactic polystyrene in solvents with different qualities has been investigated. The concentration dependence of the melting points of the beta- and delta-phases has been measured, and the experimental data have been used to construct the corresponding liquidi. The different solvents can therefore be classified in order of increasing solvent quality for polystyrene : decalin < 1,2,4-trichlorobenzene < o-xylene < 1,2-dichlorobenzene < chlorobenzene < chloroform. The stability of the beta-phase increases with decreasing solvent quality. The opposite influence of this solvent quality is observed for the delta-phase. This solvent quality induced inversion of the phase stability explains the peculiar behavior in good solvents like chloroform and the formation of thermoreversible gels in these solvents. The study of the phase behavior and the analogy with the melting behavior of mixtures of low molecular mass substances reveal the formation of an incongruent melting polymer-solvent compound. A two-step mechanism composed of a coil to helix transition followed by an intermolecular association is proposed.