Potassium selectivity was studied with film-type membranes which contained different valinomycin concentrations and were plasticized with dioctyl phthalate, didecyl phthalate, or dihexyl adipate. The results obtained are in harmony with the concept that the plasticizing solvent is one particular neutral chelating agent. It was reasoned theoretically and confirmed experimentally that a certain limiting selectivity can be attained which will not improve when an even higher valinomycin concentration is used, and which is independent of the type of solvent used. Ionometric data are used to estimate the ratios of formation constants for the valinomycin complexes of potassium, sodium, ammonium, and cesium in different plasticizing solvents.