It is the purpose of this paper to familiarize the electrical engineer with the nature and characteristics of a material on the service behaviour of which is dependent the successful and continued operation of certain of the equipment for which he may be responsible.In spite of the advent of non-inflammable, liquid chlorinated hydrocarbons, mineral oil still remains the chief dielectric or insulating liquid used in oil-filled transformers and oil switches. Mineral oil has been chosen as a liquid insulating material because it has an electric strength greater that that of air and because, where necessary, it can function as a cooling medium and so maintain the operating temperature of the equipment at a safe value.The outstanding characteristic of mineral insulating oils to which attention is directed by manufacturer and user of h.v. equipment, oil producer, and oil chemist alike, is their stability in service.Efficient operation of mineral-oil-filled electrical equipment is dependent, therefore, to a large extent upon the ability of the oil to retain substantially unimpaired, in the presence of deteriorating influences, its initially good electrical properties, its inertnesr towards the solid insulating materials that are also present in most electrical equipment in which oil is used, and its ability to effect efficient cooling.