In a previous report dealing with the significance of contact phenomena in relation to measurements in dielectrics, the E.R.A. foreshadowed further work on a range of dielectrics, with a view to studying the effect of a.c. contact impedance with different types of electrodes.The present report gives an account of this work and leads to the conclusion that, for accurate determinations of power factor, permittivity, etc., flat metal electrodes should not be used. Both mercury and graphite give satisfactory results; consequently the choice between these two may be settled by considerations of convenience.The Director of the Electrical Research Association will be pleased to hear from investigators who have occasion to apply any of the proposals outlined herein.