Recent changes in the design of aircraft engines have created a demand for lubricants that will perform satisfactorily under conditions far more rigorous than were ever experienced in the past. These increased demands cannot be met by conventional lubricants or by lubricants protected by conventional additives. Among synthetic lubricants being explored as functional fluids in this new area are the polyphenyl ethers. Polyphenyl ethers possess very good high-temperature properties. For example, bis(phenoxyphenoxy) benzenes are oxidatively stable to 500 F and thermally stable to 800 F. This paper summarizes the research aimed at improving the oxidative stability of polyphenyl ethers by the use of additives. It has been found that classic antioxidants such as amines and phenols are ineffective under extreme conditions. Chemical classes such as metal chelates, organometallics, and metal oxides inhibit oxidation of polyphenyl ethers at elevated temperatures.