Syndiotactic polystyrene (sPS) membranes containing different mole fractions of p-xylene were prepared by a solution-casting procedure. Complex formation between sPS and xylene was studied by thermogravimetric analysis and Fourier transform infrared spectroscopy. The stability and desorption behavior of the sPS-guest solvent and phase transitions were studied by differential scanning calorimetry. The formation of the delta-form complex in the presence of different mole fractions of xylene isomers was analyzed and confirmed. The mole fraction of p-xylene in the dried membrane was found to be higher than that of the corresponding mole fraction in the isomer solvent solution used for casting. This was attributed to the preferential complexing ability of p-xylene with sPS. (C) 2003 Wiley Periodicals, Inc.