Protonation and oligomerization mechanisms of pyrrole monomer were elucidated in the presence of acid such as HBF4 and Lewis acid such as AlCl3, FeCl3 in acetonitrile solutions, using experimental methods such as UV-vis and AA spectroscopy and semi-empirical AM1 calculations. It was observed that pyrrole interacts with proton and Lewis acids on similar mechanisms to produce pyrrole oligomers. The theoretical studies predicted that the both types of acids add to the pyrrole ring on C-beta atom. These studies also showed that FeCl3, which is used as oxidative agent in the chemical polymerizations, adds to the polypyrrole chain as well. (C) 2004 Elsevier Ltd. All rights reserved.