Abstract: An impregnated electrical capacitor employing a dielectric which at least in part comprises plastic foils, in which the dielectric is expanded by an impregnating agent to substantially eliminate objectionable gaps filled with air or impregnating material whereby the breakdown strength provided by the dielectric foils is not impaired. In producing such capacitor, the temperature during and after impregnation and the degree of winding compression or tightness may be varied to provide desired control of the expansion and insure complete impregnation and control of the final volume, as compared with the original, of the capacitor structure.