A channel flow cell suitable for the study of reactions at insulator/liquid interfaces is described, in which a pair of electrodes are located downstream of the solid of interest and used to monitor the release of ionic species by means of impedance measurements made at variable frequencies. Theory is given which enables the measured impedance resulting from the spatially inhomogeneous ion distribution within the flow cell, and its flow rate dependence, to be related to the rate process at the solid/liquid interface, The method is applied to the hydrolytic dissolution of solid triphenylmethyl chloride in aqueous solutions of low ionic strength and leads to the inference that the hydrolysis occurs at the solid/liquid interface rather than in bulk solution after the prior dissolution of the solid.