This paper describes research undertaken by Encore Acquisition Company and the University of Calgary regarding the flammability safety of synthetic lubricants used in Encore's compressors in their high pressure air injection (HPAI) projects in Southeastern Montana. With over 2,270 e3m(3)(ST)/d (80 MMscfd) of installed air compression capacity discharging at pressures of 31.0 to 34.5 MPa (4,500 to 5,000 psi), a critical aspect of this project is the safe and uninterrupted operation of the compressors. Experience gained by Encore and other HPAI operators shows that the reaction of high pressure air with compressor lubricants in high temperature interstage and discharge regions of the compressors can be a source of trouble (destructive overpressures), even when synthetic diester-based lubricants are employed. Using an Accelerating Rate Calorimeter (ARC), samples of fresh and used synthetic lubricants were heated in the presence 1 of air at initial pressures up to 34.5 MPa (5,000 psi). Self-heating!, rates and pressure responses were measured. The results highlighted the significant effect of pressure on: auto-ignition temperature. Most significantly, the auto-ignition; temperature of the diester-based lubricant dropped from the manufacturer's reported level of 410 degrees C (770 degrees F) at atmospheric pressure to 180 degrees C (365 degrees F) at pressures in the range of 17.2 to 34.5, MPa (2,500 to 5,000 psi). Also, the auto-ignition temperature of used (oxidized) synthetic lubricant was further reduced to values close to the operating temperature levels of the compressors. Finally, it was noted that the auto-ignition temperatures for different brands of diester-based lubricants were all very similar. The significance of this study is not only in the temperature: data, but also in the discussion of several significant changes that Encore made to the design and operation of their high pressure air compressors as a result of this study. This information will assist future HPAI operators in designing safe and reliable air compression systems.