A novel concept of a single-piece all-solid-state ion-selective electrode (SPE) is introduced. A processable conjugated (electronically conducting or semiconducting) polymer (CP) is dissolved in a cocktail containing the components used for a conventional ion-selective poly(vinyl chloride) matrix membrane. The cocktail, containing the CP, is cast directly on a solid substrate (glassy carbon), resulting in a SPE. The role of the CP is to mediate the charge transfer between the substrate and the membrane. Two soluble CPs are studied: (i) poly(3-octylthiophene) in its undoped state and (ii) polyaniline doped (protonated) with bis(2-ethylhexyl) hydrogen phosphate. Experimental results obtained for lithium-selective SPEs and calcium-selective SPEs are discussed. All the SPEs studied show near-Nernstian responses, and no redox interference is observed as long as the concentration of the CP is sufficiently low. The incorporation of a CP, particularly polyaniline, in the membrane is shown to improve the stability of the standard potential of the SPE compared to the corresponding coated-wire electrode (membrane without CP). Impedance measurements provide information about the charge transfer processes of the electrodes.