The thermodynamics of thermoreversible gelation of isotactic polystyrene in cis-decalin,frans-decalin, and 1-chlorodecane was investigated by using differential scanning calorimetry. The gel formation and the gel melting were studied as a function of various parameters such as the cooling or heating rate and the polymer molecular weight. The temperature-concentration phase diagrams were established from both cooling and heating experiments. It is found that the shape of the phase diagram is the same in heating or in cooling, but that for cis-decalin differs from the one for tron.s-decalin. The phase diagrams correspond to various types of compound: congruently melting compound in cis-decalin and compound characterized by a singular point in trans-decalin. In addition, it is shown from optical microscopy experiments and X-ray diffraction experiments that the threefold helical form reappears beyond 50% polymerconcentration in tnms-decalin but does not in cis-decalin.