Abstract: Mixedpotential theory is introduced to discuss the experimental results of open circuit potential for the autocatalytic polymerization of aniline. The open circuit potentials of Pt, Pd and polyaniline film in acidic aniline solution are much more positive than those of other metals. Owing to the electrochemical mechanism of the autocatalytic polymerization of aniline, the metal plate as substrate is endowed with a common electrode potential as for both the anodic and the cathodic half reactions. This common potential is called mixed potential. The value of the mixed potential is experimentally equal to that of the open circuit potential, and can be regarded as a criterion for the feasibility of the autocatalytic polymerization of aniline. A much positive value of the open circuit potential corresponds to an observable autocatalytic polymerization of aniline. The experimental results of the open circuit potential of various metal plates in acidic aniline solution support the electrochemical mechanism of the autocatalytic polymerization of aniline in return.