An improved dielectric system for an electrical apparatus. The dielectric system includes a polymeric film impregnated with low molecular weight polybutene having an average number molecular weight in the range of 300 to 500. The dielectric system has improved dielectrical properties and the polybutene is biodegradable which minimizes the pollution of the atmosphere if the dielectric liquid is exposed to the atmosphere. Further extended service life can be achieved by the addition of a cyclohexyl amine to the liquid dielectric.