The study of near-infrared emission has shown that singlet oxygen (O-1(2)) is generated via superoxide by the photocatalytic reaction of titanium dioxide (TiO2) dispersed in solvents. The O-1(2) was deactivated on the TiO2 surface and could not oxidize nicotinamide adenine dinucleotide, which has no affinity to the TiO2 surface. The generation of photocatalytic O-1(2) was enhanced in the phospholipid membrane, and a water soluble protein acted as an effective scavenger of the generated O-1(2). These findings suggest that O-1(2) generated by TiO2 contributes to the phototoxic effect through the oxidation of the membrane protein.