After self-assembling a p-aminothiophenol (PATP) monolayer on bare Au electrode, polyaniline (PANI) films were prepared by electropolymerization on the modified Au electrode and three forms of PANI (the emeraldine salt, the leucoemeraldine and the pernigraniline) were obtained by electrochemical, method, The cyclic voltammograms of the probe (K3Fe(CN)6/K4Fe(CN)6) for the three forms of PANI show that three different oxidized forms can be found, and all of them are stable under our experiment conditions. The potential dependence of magnitude and the spectra of the photocurrent of the PANI films were observed. When the spectra of photocurrent are displayed in a Fowler plot (the squareroot of the incident photon-to-current efficiencies versus the photon energy), the functional form appears linear, which is interpreted as indication of internal photoemission of holes or electrons from metal into the appropriate electronic band of the semiconductor or insulator. The band gap energy of insulating matrix in. the emeraldine salt is determined as 3.33 eV by the Fowler plots, The fiat-band potential., in the order of 0.63 V versus SCE, is obtained from. Mott—Schottky plots. A photoelectrochemical process based on internal photoemission in the emeraldine salt, which agrees with the model of granular metal island that assumes metallic polymer particles are embedded in the insulating matrix, is proposed.