The paper describes the results of measurements of the power factor and relative permittivity of purified, uncompounded, natural rubber within the temperature range from −50 to +40° C and within the frequency range from 50c/s to 1 Mc/s. The effect of exposure to water vapour on the dielectric properties is contrasted with the effect of a similar exposure on the properties of crude rubber.The methods of purification of the rubber and preparation of the test specimens are described, and the dielectric properties of vulcanized purified rubber are briefly indicated.The Introduction describes the nature of the rubber hydrocarbon and its natural impurities, and mentions the methods of compounding and vulcanizing rubber for electrical purposes.