Impedance spectroscopy can be used to determine the influence of several membrane parameters on the membrane resistance of anion selective CHEMFETs. The concentration of the ammonium sites in the membrane, the anion-receptor complex stoichiometry, and the polarity of the membrane matrix are of particular importance. In general the resistance of polysiloxane membranes is higher than that of PVC membranes. However, in polysiloxane membranes the membrane polarity can be influenced by the type or concentration of polar substituents on the polysiloxane chain. Polysiloxane ion-exchange membranes with 25 mol % of polar sulfone substituents exhibit the same conductance as NPOE plasticized PVC membranes. Remarkably, the membrane resistance of cation-selective polysiloxane membranes is much lower and is much less dependent on the substituents.