The pressure-volume (P-V) isotherms were measured for the poly(methy1 methacrylate) (PMMA)-methyl methacrylate (MMA) coexistence system polymerized by γ-ray irradiation at various pressures. The P-V isotherm of the coexistence system polymerized below 3000 kg/cm2was similar to that of the monomer. The specific volume of the coexistence system polymerized above 3000 kg/cm2hardly changes in the pressure range between 3000 kg/cm2and the polymerization pressure when measurements are made with successively decreasing or successively increasing pressures. The compressibility was of the order of 10−6(kg/cm2)−1in this pressure range. This indicated that the coexistence system polymerized above 3000 kg/cm2behaves like a solid in this pressure range. Above the polymerization pressure, the compressibility was of the order of 10−5(kg/cm2)−1, indicating that the coexistence system behaves as liquid. We concluded that the solid-like behavior of the coexistence system polymerized above 3000 kg/cm2is caused by a strong interaction between as-polymerized polymer chains and monomer molecules due to propagation through monomer clusters having short-range order. The present results support that MMA is aligned in short-range order at suitable pressure and temperature as reported in the preceding paper.