Response characteristics of field effect transistors covered by membranes containing neutral carriers and an organic liquid but neither a polymer nor ionic sites were determined. The commercially available 2-nitrophenyl octyl ether and dioctyl sebacate, both frequently used plasticizers for poly(vinyl chloride) ISE membranes, had impurity concentrations large enough to lead to small emf responses. Upon removal of these impurities by HPLC, no emf responses were detectable any more, which seems to be due to a negligibly small permselective uptake of primary ions into the membrane and not the high impedance of the membrane bulk. The use of membranes of the more polar nitrobenzene and 2-fluoro-2’-nitrodiphenyl ether resulted in small but transient emf responses. The absence of ionic sites led however to the failure of permselectivity and resulted in co-extraction even of the highly hydrophilic chloride ions. Liquid membranes containing neutral carriers but no ionic sites are thus not only unsuitable for ISEs due to their high impedance, but also because it is very likely that either the failure of permselectivity occurs or the concentration of primary ions permselectivity entering the membrane is too small to determine the phase boundary potential.