An apparatus for determining the specific volume of solid and liquid polymers as a function of pressure (to 2200 kg/cm2) and temperature (30–350 °C) has been constructed. It is based on Bridgman’s bellows method and makes use of a linear variable differential transformer (LVDT) mounted outside the pressure vessel to measure the length changes of the bellows. The off‐null output of the LVDT drives a digital indicator, displaying length changes with an accuracy of 0.01 mm, corresponding to a volume change of 10−3cm3. Typical samples weigh about 2 g. Volume changes of the sample are calculated from the bellows displacement using the pvT properties of the confining mercury, and by using a calibration function determined in a separate experiment. This apparatus is capable of measuring specific volume changes of polymers with an accuracy of 0.001–0.002 cm3/g. Results for a commercial polystyrene in the glassy and liquid states are given.