The nature of the plasticizer plays a pivotal role in the analytical performance of polymer membrane ion sensors. Conventional plasticizers suffer leaching or migration from the membrane and exudation, both of which could limit the lifetime of sensors based on plasticized membranes. Herein, we describe the use of polyester sebacate (PES), a model polymeric plasticizer, in the preparation of poly (vinyl chloride) (PVC) membrane ion-selective electrodes (ISEs) using valinomycin as ionophore. PVC membrane electrodes plasticized with polyester sebacate demonstrated potentiometric response characteristics that compared favorably to ones plasticized with the conventional and similarly structured plasticizer bis(2-ethylhexyl) sebacate (DOS). Increasing the content of polyester sebacate in the membrane enhanced the response and improved the selectivity of valinomycin-based ISEs toward potassium over sodium. Various methods, including electrochemical impedance spectroscopy, UV-vis spectroscopy, dark field optical microscopy, and potentiometry were employed to study the effect of plasticizer on the leaching of the membrane components and the lifetime of both DOS- and PES-plasticized membranes. PES-plasticized electrodes maintained Nernstian response and high selectivity for more than four months, an improvement over DOS-plasticized membrane electrodes. This was attributed to enhanced retention of the membrane components because of the high polymeric nature of the polyester sebacate. These characteristics suggest that polyester sebacate is a good candidate to replace the conventional plasticizers in preparing PVC membrane electrodes with longer lifetime.