The study of the phase behaviour of syndiotactic polystyrene (sPS) in o-xylene is reported. Differential scanning calorimetry, microcalorimetry, infra-red spectroscopy, wide angle X-ray scattering, transmission electron microscopy and thermogravimetric analysis have been used. The polymer chains can adopt a T2G2 helix or a planar T4 zig-zag conformation, depending on the experimental conditions. The initial concentration, the cooling rate and the thermal history strongly influence the formation and the relative contribution of the different conformations, leading to a complex phase behaviour. A decrease of the polymer concentration, and/or an increase of the scanning rate, tends to favour the formation of the helical structure. This helical conformation is always formed when a vitrified, concentrated solution is heated. The transformation of the helical conformation into the planar zig-zag conformation proceeds by a two-step mechanism of melting and recrystallization. Solvent-induced crystallization of amorphous, glassy sPS results in the formation of the helical conformation. The zig-zag conformation is always formed by crystallization from the pure melt. The formation of an incongruent polymer-solvent compound is proposed to explain the complex phase behaviour.