Purified rubber was prepared by the digestion of crude rubber or latex with water at about 190° C., followed by extraction with water and with alcohol, and drying in an atmosphere of inert gas. The digestion hydrolyzed the proteins, and the extraction removed the hydrolysis products, resins, and other impurities. The purified rubber contained about 99.5 per cent of rubber hydrocarbon. Properties of the rubber hydrocarbon at 25° C. were: density, 0.9060; refractive index, 1.5184; dielectric constant at 1000 cycles per second, 2.37; power factor at 1000 cycles per second, 0.0015; conductivity at the end of 1 minute, 2.2×10−7 mho. The electrical properties measured on 14 samples were apparently independent of the botanical source or the kind of crude rubber.