High-temperature fluid lubrication is influenced by many factors. Among these factors are the products of degradation of the fluid. Operating at bulk temperatures well below the initial decomposition point, equipment hot spots produce temperatures that result in small but significant amounts of fluid degradation. The material thus formed affects the lubricating characteristics of the fluid. Results obtained with a polyphenyl ether illustrate somewhat of an extreme of such an effect. Likewise, fluid decomposition products appear to influence lubrication with both mineral oils and silicones. Intensive study of this phenomenon is suggested as a means of achieving a sounder understanding of the mechanism of lubrication.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Rochester, New York, October 15–17, 1963.