A high-pressure autoclave with several novel design features is described. Test specimens are held in an inner vessel made of a chemically inert material, e.g. silica, thus eliminating the possibility of spurious side reactions that could alter the test-gas composition. This vessel is surrounded by a furnace and an outer steel pressure vessel. A semiautomatic gas-balancing system is used to prevent excessive differential pressures which could rupture the inner vessel. The autoclave has been designed to operate at a pressure of 44*10⁵ Pa (650 p.s.i.) and temperature of 1273K with a uniform temperature zone of about 300 mm in a reaction vessel diameter of 100 mm. Under these conditions the outer autoclave walls reach a temperature of about 450K. The autoclave is completely demountable. The specimen holder is of modular form and can accommodate several hundred small test coupons or, alternatively, a few large ones. Details of heater design, thermal insulation and control systems are given. The performance and long-term temperature stability are discussed.