A number of ion-selective electrode membranes have been investigated using a. c. impedance measurements. Membranes containing the antibiotic valinomycin have been studied in both liquid and PVC matrix form, and dibenzo-18-crown-6,21212- cryptand, and a new ionophore, COD-I were investigated in Pvc membranes. The effects of incorporating various levels of tetraphenylborate into the membranes were also investigated. For both liquid and PVC membranes, measurements were made using a four-electrode system, allowing the impedance of the membrane alone to be determined, without contributions from the cell and current-carrying electrodes, and potentiometric data were also obtained for all membranes to establish the degree of cation selectivity, and the extent of anion exclusion. A computer-controlled measuring system, and associated software was developed, to allow repeated, and accurate measurement of the impedance over long time periods. The rate of exchange between the membrane and aqueous solutions was determined for the primary ion, potassium, and the major interferent, sodium, and relative mobilities of species within the membrane were calculated. The impedance behaviour of PVC membranes was also studied prior to contact with aqueous solutions, and measurements were made on the individual membrane components. on the basis of these measurements the mechanism of cation selectivity for valinomycin-containing membrane is deduced. For liquid membranes a simple exclusion mechanism appears to operate, whilst for PVC matrix membranes a more complicated mechanism is apparent, in which all types of ion can gain access to the membrane, but the ionophore controls the overall concentration of the cationic species. For the crown and cryptand, which show poorer selectivities, the ion-exchange at the membrane/solution interface appears to be hindered, whilst the COD-I shows more similarity with valinomycin. For both liquid and PVC matrix membranes, the presence of negative sites within the membrane was found to be necessary for effective anion exclusion.