Thermal degradation of two polyol ester lubricants with the same base chemical structures from two different manufacturers at high temperature (i.e., 220 degrees C) has been studied. Gel permeation chromatography was used to identify the high molecular weight products formed during thermal stressing of these lubricants. The amount of high molecular weight products generated at different stages of the degradation (e.g., dimers, oligomers) was quantified. Several other techniques were utilized as a combined analytical approach to provide comprehensive measurement of lubricant degradation in the liquid phase. The mechanism of degradation and the formation of oxidation products have been presented. A significant difference in the extent of degradation of the two lubricants was observed using different analytical techniques.