The thermal stability properties of n-hexadecane, super-refined mineral oils, and oligomers have been measured as a function of time and temperature. The versatility of the PRL pressure cylinder thermal stability test is demonstrated. Chromatographic techniques to provide quantitative data on the products of decomposition are described. The similarity in mechanism is shown for the thermal degradation of pure hydrocarbons and mineral oils, while distinctly different mechanisms are shown for mineral oils and oligomers. Differences in the thermal degradation reactions in the gas and liquid phases emphasize the need for close control of any standard long time thermal stability test because of secondary reactions involved. The large effects of small amounts of gas and volatile products formed on the measurements conventionally used to estimate thermal degradation are discussed.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Minneapolis, Minnesota, October 18–20, 1966.