Syndiotactic polystyrene films were obtained by melt crystallization in the α form. Samples with different crystallinity were produced by varying the thermal treatment of the melt. The structural organization was analyzed by x rays, transport properties, and Fourier transform infrared measurements. The α form is characterized by chains in zigzag planar conformation; it is transformed in dichloromethane into a crystalline form having the chains in a helical conformation. The vapor activity for this transition was obtained for all the crystalline samples. The fraction of impermeable phase, derived from sorption measurements, was consistently found to be higher than the fraction of crystalline phase derived from x rays. This led to the suggestion that in these samples, besides the crystalline phase, there is also a fraction of mesophase, impermeable to dichloromethane vapors at low activity. At activity higher than a critical value this phase becomes permeable, allowing the conformational transition. At activitya= 1, in liquid dichloromethane, the crystalline phase also becomes permeable, and complete transformation into the helical structure is observed.