The temperature-concentration phase diagrams have been established for sPMMA gels prepared from toluene, chlorobenzene, and bromobenzene. In these three solvents polymer-solvent compounds are formed : one compound in toluene (two toluene molecules/three monomers) and two compounds in bromobenzene and chlorobenzene (two solvent molecules/three monomers and three solvent molecules/two monomers). In addition, in chlorobenzene the existence of a mesophase above the macroscopic gel melting is shown. Neutron diffraction and NMR investigations give further support to the existence of these compounds. NMR measurements of selective and nonselective H-1 spin-lattice relaxation rates R-1 of the solvent are consistent with the stoichiometry derived from the phase diagrams and also indicate that the mobility of the solvent in the sPMMA/bromobenzene compounds is quite high.