Radiation-induced polymerization of n-butyl and n-hexyl methacrylates (BMA, HMA) at high pressure (up to 8000 kg/cm2) and pressure-volume (P-V) behavior of the polymer-monomer coexistence systems at various pressures were studied by dilatometric method. The Van't Hoff plot of the rate of polymerizations broke at 3500 and 4500 kg/cm2for BMA and HMA, respectively. (first breaks). The activation volumes were about -24 cm3/mole for both monomers below the pressure giving the first breaks. The pressure dependences on the rate of polymerizations were small in the pressure ranges of the first break pressures to 4500 kg/cm2for BMA and 5500 kg/cm2for HMA (second breaks), and above the second break pressures the rate of polymerizations increased with constant activation volumes with increasing pressure. The P-V isotherms of the coexistence systems polymerized below 3000 kg/cm2were the same as those of the monomers, and the coexistence systems polymerized at 3800 (BMA) and 4200 kg/cm2(HMA) showed solidlike features in the pressure range of 3000 kg/cm2to the polymerization pressures. The coexistence systems polymerized at 5500 (BMA) and 6500 kg/cm2(HMA) showed solidlike features in the range of 4500 kg/cm2to the polymerization pressure and 5500 kg/cm2to the polymerifation pressure, respectively. The pressure of 4500 kg/cm2for BMA and 5500 kg/cm for HMA corresponded to the pressure giving the second break in the Van't Hoff plot. We concluded from these results that the solidlike features were due to a strong interaction between as-polymerized polymer chains and monomer molecules similarly to the case of the PMMA-MMA coexistence system, and that the ester chains of BMA and HMA play an important role in alignment of monomer molecules above 4500 (BMA) and 5500 kg/cm2(HMA).