Solid contact potassium-selective electrodes with the internal ion-to-electron transduction layer composed of plasticized poly(vinyl chloride) (PVC) and 2-20% (m/m) of polyaniline (PANI) nanoparticles, with the mean particle size of 8 nm, have been studied in this paper. UV-vis measurements in pH buffer solutions between pH 0 and 12 show that the electrically conducting emeraldine salt (ES) form of PANI has exceptionally good pH stability. Membranes of PANI nanoparticles were mainly in the ES form even at pH 12, in contrast to electrochemically prepared PANI(Cl) films, which are converted completely to the nonconducting form already at pH 6. Long-term UV-vis measurements with the PANI membranes in contact with aqueous buffer solution at pH 7.5 showed no degradation of the ES form. The PANI nanoparticles are homogenously mixed in the PVC-based solid contact (SC) layer. Only the uppermost part of the SC layer is to a minor extent dissolved in the outer potassium-selective PVC membrane. This enabled the preparation of geometrically well-defined inner SC layers, thus improving the reproducibility of the solid contact electrodes and resulting in good mechanical strength between the inner and outer membranes.