The mechanism of pigment film formation by photocatalytic oxidation of surfactant is investigated. The method utilizes an aqueous dispersion consisting of organic pigment, redox-active surfactant, photocatalyst, and sacrificial agent, Photoirradiation to the dispersion through a photomask gives a pigment film exclusively on the unilluminated (masked) area of a transparent conductive substrate, contrary to our expectations. In order to understand this anomalous deposition behavior, the effects of deposition conditions (substrate materials, light intensity, cell structure, dispersion components, application of bias voltage, etc.) on film formation are investigated. As a result, it is revealed that a photocurrent flowing from the unilluminated to the illuminated area through a substrate is generated, and that pigment film is deposited on the unilluminated area, where anodic oxidation of pigment-surfactant aggregate occurs. Further electrochemical and particle size analysis measurements reveal that the pigment film formation proceeds in the following successive steps : (1) photoexcitation of photocatalysts, (2) oxidation of surfactants by excited catalysts in the illuminated area, (3) flocculation and precipitation of pigment particles in the illuminated area, (4) oxidation of surfactants in the unilluminated area by excited catalysts through a conductive substrate, and (5) deposition of pigment particles in the unilluminated area, leading to pigment film formation (negatively patterned film).