This guide recommends tests and evaluation procedures, as well as criteria and methods of maintenance for Less Flammable Hydrocarbon (LFH) transformer insulating fluids. These are also known as High Molecular Weight Hydrocarbon (HMWH) fluids. HMWH dielectric fluids are hereby defined as those fluids having an ASTM D92 fire point of 300 °C or greater, being primarily composed of, but not restricted to, hydrocarbons. These fluids meet the requirements of ANSI/NFPA 70-1990, National Electrical Code [1]1 Article 450-23 as a less flammable dielectric for use in indoor applications when used in properly designed and installed transformers. Ester- and silicone-based fluids are not covered by this guide. Methods of reconditioning LFH fluids are also described. It is not the intent of this guide to address retro-filling transformers with LFH fluids. Any such substitution of fluids should only be performed after consultation with the manufacturers of fluid and equipment. Where instructions given by the transformer or fluid manufacturers differ from those given in this guide, the manufacturer's instructions are to be given preference.