A method of efficiently eliminating impurities, particularly of low molecular weight and low concentration, from electric insulating liquids by means of applying an electric field to the electric insulating liquid to charge the contaminant particles therefrom and then contacting the thus-treated liquid with at least one absorbent to remove the charged contaminant particles from said liquid by means of an absorbent having ion-radicals in its structure, such as zeolite, and then eliminating the rough particles therefrom by means such as a percolator, centrifuge or percipitation tank (7). In certain cases wherein a large amount of contaminants which interfere with the insulating properties of the electric insulating liquid are present, the electric insulating liquid is first heated and subsequently contacted with an absorbent prior to the step of applying an electric field to preliminary remove contaminants therefrom.