The dependence of the specific volume of a commercial sample of poly(4-methylpentene-1) (Mitsui TPX, RT-20, abbr. PMP) on temperature (30°–320°C) and pressure (0–2000 kg/cm2) has been determined. Results are reported in tabular form and as approximate fits, making use of the Tait equation. The data show that the crystalline melting transition of this type of PMP is completed at 235°C under zero pressure and gives indication of a glass transition temperature Tg at about 20°C at p = 0. Its approximate pressure dependence is given by dTg/dp ≈ 0.015°C kg−1 cm2. The zero pressure results on the melting and glass transitions are in agreement with DTA results. The p-v-T data, quenching experiments, and a determination of the crystalline unit cell (tetragonal, a = b = 18.70 Å, c = 13.54 Å) confirm earlier work indicating that the room-temperature crystalline specific volume of PMP is greater than the amorphous specific volume. This unusual density behavior persists to a temperature of 50–60°C at p = 0 and to temperatures as high as 230–240°C under a pressure of 2000 kg/cm2.