The ac impedance characteristics of an ion-selective membraneinsulator semiconductor (MIS) structure designed for chemical sensing applications have been modelled quantitatively for a frequency range from about 5 Hz to 40 kHz. When plasticized poly(vinylchloride)-based K+-selective membranes, nominally 100 mu m thick, are used with 45 nm of Si3N4 and 50 nm of SiO2 on a Si electrode, the impedance characteristics are dominated by the membrane above 100 Hz. Capacitance-voltage curves show a complex potential dependence below this frequency. The bulk resistance of the membrane contributes significantly to the equivalent capacitance below 1 kHz. Experiment and theory show that sensors based on ac or impedance measurements of changes in the Si space charge will be subject to significant errors if the membrane resistance is also a function of the solution conditions.