The coking characteristics of polyphenyl ether fluids were studied using the AFAPL Static Coker and the Micro Carbon Residue Tester. Test parameters were varied, and the effects of sample size, temperature, time, and test surface material on the amount and type of deposit formed were determined. The effects of oxidative degradation of the fluids and the presence of wear debris and antioxidants were also determined. Several used lubricants were obtained to identify differences in coking tendencies between the new and used polyphenyl ether lubricants. Deposit morphology, melting point, and solubility were determined to provide information about the types of deposit formed.Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990