Using polymerization solutions containing 0.1 M aniline and 1 M HCl or 1 M HClO4, which happened to give the same pH (0.2), we investigated the effects of different counter-anions on the primary and higher order structures as well as on electrochemical responses of the obtained polyaniline (PAn) films. From time developments of polymerization current and u.v.-visible absorption spectra as well as the results of atomic force microscopic (AFM) observation, we found that in both media the higher order structure of the PAn films developed in almost the same manner from grainy to fibrillar texture and finally to bundles of the fibrils. The primary structures as judged from the spectra were not very different, but the structural size was somewhat larger for the films prepared in HClO4 than the ones prepared in HCl. The apparent acid dissociation constants of the resulting PAn films were again not so different, reflecting little differences in their primary structures. However, the electrochemical responses of the former were much slower than those of the latter, especially when the transition from the oxidation to reduction form was compared, reflecting differences especially in the structural size and compactness of their higher order structures.