The characterization of polyindole made by anodic electropolymerization of indole monomer in dichloromethane is reported based on scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results on SEM show that the granules in the polymer film are arranged in an ordered manner. The domain of the nuclei in the film made under potentiostatic condition is much smaller as compared to the domain of the nucleation in the polymer film made under potentiodynamic condition. The DSC data shows that the polymer is stable at higher temperature. A solid-state potassium ion-selective electrode (ISE) using polyvinyl chloride (PVC) matrix membrane impregnated with valinomycin at the surface of the polyindole-modified electrode is reported. Neutral-carrier impregnated PVC membrane is formed by casting over the surface of the polyindole-modified electrode. The resulting ISE without the incorporation of polyanions in the polyindole film shows high selectivity to K+ with negligible drift of the baseline potential with a slope of 59 mV/dec within Nernstian behavior. The detection limit of the potassium ion sensor is 6 x 10(-5) M with a wide linearity over almost 5 dec. Typical responses of the neutral-carrier impregnated PVC membrane over the polyindole-modified electrode to potassium ion is reported.