The term askarel generally describes a widelyused, broad class of non-flammable synthetic halogenated hydrocarbon insulating liquid. As treated in this Guide it applies solely to askarel used in transformers, reactors, and accessory equipment operated at power frequencies. Certain essential properties must be retained if askarel is to perform reliably its dual role of electrical insulation and heat-transfer agent. It must have adequate dielectric strength to withstand the electric stresses imposed in service. It must retain a sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired. The dielectric losses should not become excessive. It should not be allowed to become so deteriorated or contaminated that it adversely affects other materials in the apparatus. This guide attempts to assist the power equipment operator in his efforts to maintain askarel in serviceable condition. It recommends standardized tests and evaluation procedures. Methods are outlined for reconditioning and reclaiming askarel whenever necessary