Toluene vapour sorption behaviours in syndiotactic polystyrene (SPS) film were measured at 25 degrees C by the quartz crystal microbalance method. The sorption isotherm of this system was concave toward the pressure axis below the activity of toluene vapour (p/p(0) = 0.3), indicating a strong interaction between toluene vapour and sorption sites in the mesophase consisting of TTGG conformation. On the other hand, the sorption isotherm of toluene vapour in atactic polystyrene film showed a normal good solvent type and could be described by Flory-Huggins thermodynamics. In addition, the structural changes in SPS film by toluene vapour sorption were characterized by Infrared spectroscopy. The absorbance at 933 cm(-1), which is assigned to TTGG conformation, was found to be constant below p/p(0) = 0.3; however, it increased above p/p(0) = 0.3 and was saturated above p/p(0) = 0.6. These results showed that toluene vapours penetrated into the pre-existing sorption sites composed of TTGG conformation to form the clathrate structure (delta form) below p/p(0) = 0.3. Furthermore, in the range of pipe = 0.3-0.6, the sorption of toluene vapour could induce a large increase in the ordered structure (delta form), i.e., TTGG confirmation. The changes in the non-clathrate structure with toluene (alpha form) composed of TT conformation, could be characterized by the absorbance at 1222 cm(-1). It was confirmed that the alpha form could be also increased by toluene vapour sorption above p/p(0) = 0.4. The sorption enthalpy of toluene vapour (Delta H-8), estimated from sorption isotherms showed a negative value below p/p(0) = 0.6, but showed a positive value above pipe = 0.6, even though the polymer-good solvent pair. This result also supports the structural organization in SPS film by toluene vapour sorption.