I N THIS J E T and space age more and more emphasis is on thermal stability. For lubricants and hydraulic fluids it is apparent that other properties such as viscosity, pour point, oxidative stability, and vapor pressure are important, but such properties are more amenable to improvement by additives and minor structural modifications than is thermal stability. There is no systematic recording in the chemical literature of the thermal stability of organic compounds under the same conditions. Many of the chemical literature data on thermal stability are based on the stability of a material passing through a hot tube and on observations made upon distillation. This paper reports on a comparison under the same conditions of the thermal stabilities of over 100 organic compounds in 12 chemical classes and the relation of these data to structure. The test method and a large portion of the data reported here resulted from a search for materials with a minimum thermal stability of 700" F. (5).