An autocatalytic polymerization occurs during electrolysis of 0.20 M aniline solutions of more than 0.6 M in HCl, and at potentials more positive than 0.80 V vs the Ag/AgCl/saturated KCl electrode. The evidence for autocatalytic polymerization is that the current increases with time at constant potentials above 0.80 V; the anodic peak potential shifts towards negative potentials by about 80 mV from the first to the second scan and the current increases with the number of potential cycles during potential cycling. The results from the rotating ring-disk electrode show that a soluble intermediate is generated at the disk electrode during electrolysis of the above solutions at potentials above 0.80 V, the soluble intermediate initiated the autocatalytic polymerization of aniline; and two intermediates were collected at the ring electrode in solutions with pH values above 4.65, which cannot catalyze the oxidation of aniline. The collection efficiencies of the intermediates at the ring electrode increase with increasing acidity, so the soluble intermediates may be the radical cations.