Radiation-induced polymerization and pressure-volume (P-V) measurements of acrylonitrile (AN) were studied up to 8000 kg/cm2in the temperature range of 6–72°C. P-V isotherms of AN have several small breaks, A phase diagram of AN was obtained from the breaking pressures and temperatures. Liquid phases were named LI, LII, and LIII, from low to high pressure. The polymerization behavior and volume contraction on polymerization changed in LI, LII, and LIII. The difference in entropy between original and activated states decreased with increasing pressure at the same phase, but increased with phase change in LIto LIIand LIIto LIII. It was concluded from these results and from IR data on PAN that molecular packing of AN in liquid changed in LI, LII, and LIII. In LIIand LIII, AN molecules aligned in a less suitable geometry for polymerization than in LI.