A fluorescence probe technique was applied to get information on the role of the solvent in the isotactic polystyrene (iPS-decalin) thermoreversible gel system. Fluorescence anisotropy values of naphthalene, 1-methylnaphthalene, 1,5-dimethylnaphthalene, and anthracene, which were dispersed in the iPS gel, were examined in detail while the concentration of iPS was changed. Probe molecules larger than naphthalene in molecular size were found to be mobile in the iPS gel, indicating that solvent molecules are mobile in the area where solvent gathers and that motion of solvents near the interface with iPS is not limited so much. However, naphthalene, the smallest probe molecule in the present work, showed higher values of fluorescence anisotropy in the iPS gel, and this effect increased with an increase in the concentration of iPS. From these results we conclude that there exists a free volume where naphthalene molecules can break in among iPS helical rods in an area where the iPS chains associate. This means that the possibility is very high for decalin molecules also to be incorporated into a polymer-solvent compound and that their motion is limited, because cis-and trans-decalin molecules are smaller than naphthalene in size. By means of this fluorescent method there is no large difference detectable among iPS gels formed with cis-and trans-decalin at concentrations below 20% (w/w).