A K+-sensitive capacitive electrolyte-membrane-insulator-semiconductor (EMIS) sensor has been developed. The sensor utilizes a valinomycin-containing PVC-based membrane with different contents of plasticizer. This new type of sensor has been investigated in terms of its intrinsic characteristics, like impedance behavior, capacitance/voltage characteristics and frequency dependence. The optimized working conditions of the sensor and various membrane compositions have been studied with regard to the sensitivity performance in different electrolytes. The possibility of future applications for the measurement of extracellular potassium-ion concentrations are discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.