By coating an electroactive polypyrrole (PPy) film electrode with a plasticized poly(vinyl chloride) (PVC) membrane containing valinomycin, an all-solid-state potassium-selective electrode was fabricated. The polymerizing conditions of the PPy film were optimized for stable ionic responses. The electrochemical characteristics of these electrode systems were evaluated by an a.c. impedance method with respect to the mechanism of potential drift. The steady-state response over several days, which depended on the electrode configuration, was related to the impedance behavior of the electrode systems.