The effect of the SO4(2-) anion on both potentiometry and voltammetry with poly(pyrrole) films doped with chloride or hexacyanoferrate(II/III) ions has been studied and compared with the responses obtained in KCl solutions. Strong interference was observed when the SO4(2-) to Cl- molar ratio was 1:1000, leading to a decrease in polymer film electroactivity at higher sulfate concentrations. The effect is attributed to incorporation of the divalent anion into the polymer film structure, a hypothesis supported by analysis of the composition of the elemental film.