The properties and behaviour of some silicone materials of interest to power engineers have been examined, using mainly British Standard test methods, or modifications of them, so that comparison with conventional materials can readily be made. These, together with the results of accelerated life-tests on motors employing silicone resins as bonding and impregnating media, show that such materials are capable of working at temperatures at least 50° C higher than those permissible for conventional insulants.Many of the materials also possess properties, other than that of thermal endurance, which make them eminently suitable for specific applications. Some of these are described, together with some of the difficulties encountered during limited experience in using the materials.