SINCE the Second World War, no lubrication problem has offered a greater challenge to chemists than that posed by the aircraft gas turbine engine. Mineral oils, which for many years had provided satisfactory lubrication of piston‐engined aircraft, had obvious limitations when considering jet engines, and more than ten years ago the need for new lubricants was realized. The requirements were improved high temperature performance coupled with low volatility, fluidity at low temperatures, and high load‐carrying capacity. The requirements of satisfactory lubricants for aircraft gas turbine engines were discussed as long ago as 1947 by Williams, who proposed certain tentative test methods and pointed out the limitations of the mineral oils currently in use. At that time research on potential synthetic lubricants had begun both in the U.K. and in the U.S.A., and during the next four or five years bench engine tests were carried out, followed by flight trials in aircraft.