When a sinusoidally varying p.d. is applied between the electrodes of an ion chamber subject to ionizing radiations, the motion of the charge carriers and the form of the current in the external circuit depend upon the applied voltage and frequency, the gas filling and the flux level.A brief analysis of such conditions is made with space-charge effects neglected and is then modified semi-empirically to take account of space-charge phenomena. In the course of this analysis a definition of ‘saturation’ as applied to chambers using alternating polarizing voltages is proposed, and a series of experiments is described showing how variations in the aforementioned parameters affect this condition. Throughout the tests a standard A.E.R.E. RC1 ion chamber was used and was irradiated in the reactors Gleep and Bepo.