It is shown that anion-selective electrodes with low concentration of quaternary ammonium sites exhibit an apparent response and selectivity to HCO₃- and HPO₄^2- ions in aqueous solutions. It is, however, detected that none of these ions is the potential-determining, and the electrodes actually respond to OH^-. The selectivity to OH^- is due to unidentified anionic impurities in the plasticizer, and vanishes with aging of the electrodes. Additional purification of the plasticizer eliminates the selectivity to OH-, which, however, is restored after intentional addition of negatively charged sites. Quaternary ammonium sites with longer alkyl chains provide higher selectivity to OH- suggesting that this selectivity originates from steric hindrances against bigger anions.