The structural features and the thermodynamics of melting of thermally reversible gels of poly(diphenylsiloxane) (PDPhS) formed from 1.5 to 85 wt % solutions in diphenyl ether (DPhE) have been studied by means of differential scanning calorimetry, X-ray diffraction, and electron scanning microscopy. It was established that gelation occurs as a consequence of the crystallization of PDPhS that leads to a specific spherulitic morphology; i.e., overlapped spherulite-like superstructures, which are composed of lamellar crystallites, form a three-dimensional skeleton filled with the solvent. The phase diagram of the PDPhS/DPhE system was constructed. Its particular feature, resulting from the crystal-mesophase transition in the undiluted PDPhS, is interference between the crystal-isotropic solution and the thermotropic mesophase-isotropic solution equilibrium. The equilibrium temperature T(m)degrees and the heat DeltaH(m)degrees of a virtual crystal-isotropic melt transition in PDPhS in the absence of solvent were estimated (280 degreesC and 10.34 kJ mol(-1)) through use of the Flory equation when taking the melting temperatures of the annealed gels as the equilibrium melting points of PDPhS crystals in the presence of DPhE. The isotropization temperature (similar to560 degreesC) of the mesophase, which cannot be measured experimentally because of PDPhS thermal degradation, was assessed by extrapolation of the mesophase-isotropic solution boundary curve on the phase diagram of the PDPhS/DPhE system to the 100% polymer concentration. Phase diagrams expected for the system solvent-crystalline flexible/semirigid-chain polymer, exhibiting a crystal-mesophase transition, were schematically considered and compared to that of the PDPhS/DPhE system, On the basis of this consideration, the conclusion was drawn that the mesophase behavior of PDPhS is connected, at least in part, with a relatively high stiffness of its macromolecules.