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This paper presents the most significant results of multiyear research carried out in order to evaluate the thermal endurance properties of insulating materials. The thermal life model, proposed by Dakin, has been analyzed and compared with the experimental data; the importance of the selection of properties and their end points (failure criteria) on the shape and location of the thermal endurance graph, has been emphasized and new procedures have been proposed which allow a single thermal endurance graph to be obtained (even when many properties are measured to evaluate aging) and lower test temperatures to be used (without prolonging the test time). By these procedures a reliable and objective value of the temperature index can be determined even though the thermal endurance graph is not linear; this is particularly useful for comparing insulating materials. The experimental results reported in the paper are pertinent to thermosetting and composite materials, which have been exhaustively investigated.