Copolymerization of aniline (AN) and para-phenylenediamine (PPDA) was electrochemically performed by cyclic voltammetry on IrO2-coated titanium electrodes in 0.5 M H2SO4. The cyclic voltammograms, with and without a middle peak at about 580 mV, can be produced by controlling the PPDA concentration in the aniline solution during polymer preparation. The peak at about 580 mV corresponds to the para-aminophenol/benzoquinoneimine (PAP/QI) redox couple and cross-linking sites. The mass of polymer deposited on an IrO2-coated titanium electrode is correlated with the polymer anodic peak current, which allows the rates of polymer deposition to be monitored by increases in the anodic peak current at various PPDA concentrations. SEM photographs show that the morphology of the polymer film depends dramatically on PPDA concentration. Stability test information can be used in generating an effective index to discern between the crosslinking and the PAP/QI reactions induced by PPDA. The linear relationship between the second redox process potential (E1/2), corresponding to the oxidation and reduction between polaronic emeraldine and pernigraniline in PPDA-modified PANI films, and pH, possesses a slope of about -120 mV/pH.