A general theory has been developed for voltammetric ion sensing of cations and anions based on the use of an electrode coated with a membrane containing an electroactive species, an ionophore, and a supporting electrolyte dissolved in a plasticizer. In experimental studies, a membrane coated electrode is fabricated by the drop coating method. In one configuration, a glassy carbon electrode is coated with a poly(vinyl chloride) based membrane, which contains the electroactive species, ionophore, plasticizer and supporting electrolyte. In the case of a cation sensor, ionophore facilitated transfer of the target cation from the aqueous solution to the membrane phase occurs during the course of the reduction of the electroactive species present in the membrane in order to maintain charge neutrality. The formal potential is calculated from the cyclic voltammogram as the average of the reduction and oxidation peak potentials and depends on the identity and concentration of the ion present in the aqueous solution phase. A plot of the formal potential versus the logarithm of the concentration exhibits a close to Nernstian slope of RT/F millivolts per decade change in concentration when the concentration of K+ and Na+ is varied over the concentration range of 0.1 mM to 1 M when K+ or Na+ ionophores are used in the membrane. The slope is close to RT/2F millivolts for a Ca2+ voltammetric ion-selective electrode fabricated using a Ca2+ ionophore. The sensor measurement time is only a few seconds. Voltammetric sensors for K-, Na +, and Ca2+ constructed in this manner exhibit the sensitivity and selectivity required for determination of these ions in environmentally and biologically important matrixes. Analogous principles apply to the fabrication of anion voltammetric sensors. © 2010 American Chemical Society.