Nanocrystalline TiO2 (rutile and anatase) film electrodes usually show anodic photocurrents in aqueous electrolytes. Detailed photoelectrochemical studies have revealed that they also show cathodic photocurrents under particular conditions, e.g., in alkaline solutions containing dissolved oxygen under illumination from the TiO2-film side at short wavelengths such as 300 nm, The cathodic photocurrents appear in a potential region about 0.5similar to0.9 V more positive than the flat-band potential (U-fb) of single-crystal n-TiO2 electrodes. It is shown that the appearance of the cathodic photocurrents can be attributed to efficient electron transfer from the conduction band of TiO2 particles to chemically adsorbed oxygen molecules, the density of which is largely increased in alkaline solutions through a charge-transfer interaction between surface anionic groups such as Ti-O- as an electron donor and oxygen molecules as an electron acceptor. The dependences of the cathodic and anodic photocurrents on the illumination wavelength, the illumination direction, the electrode potential, and the crystal form of TiO2 particles are discussed in relation with the photocatalytic activity of particulate TiO2 films.