The electrochemical and spectroscopic identification of the structure of the electroactive polymer poly( o-phenylenediamine), (PPD), which is prepared on electrode surfaces as a thin coating film by electropolymerization of o-phenylenediamine in an acidic aqueous solution, was carried out, as well as comparison with the results obtained for the chemically prepared polymer similar to the PPD, o-phenylenediamine and some related compounds. As a result, it is demonstrated that the structure of the PPD is essentially different from those of the electroactive polymers prepared by electropolymerization of aniline and its derivatives and that the PPD is a ladder polymer with phenazine rings.