An investigation has been carried out into the effect of ionic and other additives on the power factor of oil-impregnated paper, with the object of relating the power factor of the impregnated paper to the power factor of the impregnant. For power-factor measurements, an improved test cell was used which was designed to operate at high stresses over a wide temperature range. it was observed that there was a variable relationship between the power factor and the volume resistivity of oils depending upon the type of additive. There was no correlation between the power factor of impregnated paper and the power factor of the impregnant, but a definite relationship was found between the power factor of impregnated paper and the volume resistivity of the impregnant, this relationship being independent of the type of additive.