Electrochemical formation and overoxidation of polyaniline (PAn) were investigated in solutions containing different anions. The anions promoted PAn growth in the order, H2SO4 > HCl > HClO4. The growth rate of PAn in the presence of two different types of anion was governed by the anion with lesser promoting effect. The addition of a small amount of HClO4 into the H2SO4 solution greatly inhibited the initial monomer oxidation and the growth of the polymer. Similar promoting/depressing effects of anions were observed on electrochemical degradation of PAn films. The effects of anions were possibly related to the stability of complexes between the anion and protonated monomer, and the anion and protonated PAn chains.