The steady photoelectric response of an azopyridinium compound, (CH3)(2)NC6H4-N=N-C5H4NC18H37I (AI), monolayer modified SnO2 electrodes prepared by the Langmuir-Blodgett technique has been investigated using a conventional photoelectrochemical cell. Under ambient condition, a cathodic photocurrent is observed when the AI-SnO2 electrode is illuminated by white light (the LR was filtered). The action spectrum is coincident with the absorption of the electrode in the region around 450 nm, indicating the AI aggregates in LB film are responsible for the photocurrent. Dependencies of the photocurrent on some factors have been studied, which may change the magnitude of the observed photocurrent, namely, the light intensity of the irradiation, the concentration of the donor or acceptor in electrolyte solution, bias voltage, and the saturation degree of O-2 or N-2. Based on the changes of absorption spectra of AI in benzene solvent before and after irradiation, the electrochemical behaviors of AI in LB films are studied and the calculations of bond length, charge distribution, and total energy of different state for an analogue of AI, (CH3)(2)NC6H4-N=N-C5H4N+CH3 (AM), were performed by using MINDO/3 method. The photocurrent generation is believed to result from photoinduced tautomerism upon irradiation.