Abstract: A calorimeter for measuring the latent energy of a sample at a pressure of at least 1,000 bars has a test chamber enclosed in a substantially adiabatic enclosure. It has thick walls able to withstand the very high pressure, and is connected by capillary tubing to means for holding it at a set pressure. The test chamber and a reference body which is thermally equivalent to the chamber are surrounded by respective adiabatic jackets and symmetrically arranged in an isothermal enclosure which is held at a set temperature. A differential gas thermometer has bulbs in contact with the test chamber and with the reference body, and a two-input differential pressure measuring unit has each input connected by a capillary tube to one of the bulbs. The pressure measuring unit has a manometer capsule with a respective input on each side of a flexible membrane, a capacitive pickup for sensing the position of the membrane, and electronic readout means. It is enclosed in a thermally insulative case.