A high pressure apparatus for determining PVT data of liquids and plastic crystals and volume changes which might accompany phase transitions is described. In the present investigation the PVT data of acetonitrile have been determined from +40° to −50 °C and up to 3000 bar, including the volume changes due to the melting and to a solid–solid transition. The procedure for calibrating this apparatus is discussed. The accuracy of the PVT data is supposed to be better than 0.1%, both for the liquid and the solid state.