This paper describes theoretical and experimental work on the dielectric loss of films of insulating liquids so thin that the motion of ions under an alternating field is limited by the boundaries of the film. Films of this thickness normally occur in porous, impregnated insulation, and lead to a dielectric loss angle which varies greatly with the applied stress.In Section (2) of the paper approximate equations are established for the dielectric loss of such a thin film, as a function of the applied stress. It is also shown that measurements on thin films afford a new method of deducing the ionic concentration, radius and mobility.In Section (3) experimental work is described upon films of an insulating liquid (trichlorbenzene)as thin as 0 03 mm. Such films show a large variation of loss angle with stress, agreeing closely with the predicted variation, except in one or two cases where the nature of the ionized impurities appeared to be abnormal. Values of ionic radius and mobility deduced from the equations of Section (2) check well with values known from electrolytic measurements. It is concluded that experiment confirms the theory put forward, except at very low stresses, where assumptions made in the theory are no longer valid.In Section (4) the equations of Section (2) are applied to an actual impregnated dielectric with non-uniform pore sizes, and it is shown that provided account is taken of the distribution of pore sizes, the theory will account satisfactorily for the variation with stress of the loss angle of condensers impregnated with liquids containing ionized impurities. Values of ionic radius and mobility deduced from these results are also of the right order of magnitude, but are less accurate than those obtained in Section (3), owing to lack of knowledge of the exact distribution of pore sizes.Detailed conclusions are given at the end of the paper.