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Power factor, loss factor, dielectric constant, volume resistivity, surface resistance, and dielectric strength of epoxies, varying in viscosity and reactivity and cured with several types of hardeners, have been determined and form the basis for this report. The effect of cure, temperature, and frequency on electrical properties also is presented. The importance of proper selection of resin, hardener, and resin/hardener ratio for optimum properties is emphasized. In most illustrations, the ratio of epoxy/hardener is used that exhibits optimum heat distortion. The heat distortion point of an epoxy system is shown to provide a good indication of the effect of temperature on power factor and dielectric constant. Brief reference is made to a number of field applications of epoxy electrical insulation.