The operation of ion-sensitive field effect transistors (ISFETs) in combination with solvent polymeric membranes is described and their chemical performance is discussed. It is shown that the stability of these devices can be improved by the introduction of an intermediate electrolyte solution with a fixed ion concentration between the gate insulator and the membrane. Using the same method, interferences from carbon dioxide and organic acids can be diminished. The encapsulation method with glass was found to provide a suitable method for hydrogel and membrane fixation, and to ensure a long lifetime for the sensor.