Abstract The photoreduction behavior of polyaniline films containing TiO 2 particles (PANI + TiO 2 films) in methanol solution has been studied using the quartz crystal microbalance (QCM) technique. When a protonated PANI + TiO 2 film in an oxidized state is illuminated in the presence of methanol, the amount of electrolyte anions included in the film as charge compensators decreases monotonically with illumination time. Illumination of a deprotonated form in an oxidized state of the film results in incorporation of the electrolyte anions into the film at an initial stage, but the incorporated anions are eliminated from the film on further illumination. These results support the reaction mechanism of the photoreduction of the PANI + TiO 2 film which has been proposed previously: positive holes generated in illuminated TiO 2 oxidize methanol, resulting in protons which protonate the deprotonated polyaniline such that the electrochemically inactive film becomes electrochemically active and is then r educed by photoexcited electrons. The rates of the photoreduction, protonation and deprotonation of the PANI + TiO 2 film were evaluated by analyzing transient changes in the QCM signals caused by the illumination. The rates of both protonation and photoreduction of the film were proportional to the illumination intensity, whereas no appreciable influence was observed for the deprotonation reaction of the protonated polyaniline.