In recognition of the general need for a more exact method for determining thermal stability of plasticized polyvinyl chloride compounds used for electrical insulation, it should be pointed out that existing methods are inadequate. Amongst the electrical and mechanical properties of an insulating material at elevated temperatures underlies a fundamental property of thermal stability or resistance to degradation. There is no quick way of exactly determining such thermal stability of an electrical insulating material, and the “ten degree rule” can only be used as a basis of making a guess on the probable life of insulation.