The thermoreversible gel formation of syndiotactic polystyrene (SPS) was studied in detail. Even if SPS solutions with high concentrations did not form gels, there were the cases where SPS formed gels at lower concentrations of SPS, which favor the formation of a polymer-solvent molecular compound. Sixty-two solvent compounds were examined in regard to whether they could form SPS gels. All the compounds whose molecular volume is between 69 and 153 angstrom(3) and whose Fedors' solubility parameter value is between 8.8 and 123 (cal/cm(3))(1/2) were found to produce SPS gels as solvent. Since a solvent molecule that can form SPS gel can be a guest of SPS delta co-crystalline phase, it has become convenient to find functional molecules able to co-crystallize with SPS by taking into account these conditions of gelation solvent molecules. (C) 2015 Elsevier Ltd. All rights reserved.