The thermal stability of polyol-ester-based thermal pastes is evaluated by weight loss, viscosity and thermal contact conductance measurements. A high degree of thermal stability has been attained by using a half-hindered phenolic primary antioxidant and a thiopropionate secondary antioxidant. By using either carbon black or boron nitride particles as the solid component, a thermally conductive paste with a high degree of thermal stability has been attained. The antioxidants cause the residual weight (excluding the solid component) after oven aging at 200 degrees C for 24 h to increase from 36 to 97 wt.%. They cause the viscosity not to increase upon heating and they reduce the thermal cracking tendency. They do not affect the thermal contact conductance measured across mating surfaces that sandwich the paste. The use of a fully-hindered phenolic primary antioxidant is less effective. Both carbon black and boron nitride serve as antioxidants in the presence of either primary antioxidant or secondary antioxidant at 200 degrees C, though, in most cases, they degrade the thermal stability in the presence of both primary and secondary antioxidants, particularly at 220 degrees C. Below 180 degrees C and in the presence of primary and secondary antioxidants, boron nitride is particularly effective in promoting the thermal stability. Boron nitride paste shows an estimated lifetime of 19 years at 100 degrees C, compared to 1.3 years for the carbon black paste, and 0.10 year for commercial polyol-ester-based Arctic Silver 5. Carbon black paste has a lower tendency for cracking after heating than boron nitride paste, due to the low volume fraction of the solid component.