The electropolymerization of ortho-phenylenediamine (o-PD) in 0.10 M H2SO4, 0.10 M H2SO4 + 0.40 M Na2SO4, and 0.50 M H2SO4 solutions was studied using an electrochemical quartz crystal microbalance. The poly-(ortho-phenylenediamine) (PoPD) films were formed during continuous potential cycling between -0.40 V and +0.95 V versus SCE. The decrease in the resonant frequency of the quartz crystals during electropolymerization of phenylenediamine demonstrated that the deposition of the polymer films occurred mainly in the potential range where the monomers were anodically oxidized. Dramatic changes in the resonant frequency and the resonant admittance were observed during the redox processes at potentials below +0.20 V. At least two kinds of deposits could be distinguished in the PoPD films. The surface concentration of the electroactive sites of the polymer prepared in 0.10 M H2SO4 and in 0.10 M H2SO4 + 0.40 M Na2SO4 solutions were much higher than that prepared in 0.50 M H2SO4. The acidity of the solution rather than the concentration of the anions of the electrolyte was found to have a strong influence on the electropolymerization processes. The results were discussed in the light of the effects of the solubilities of oligomers in solutions during the electropolymerization processes and the changes in the structures of the initially formed polymer films.