The production and decay of singlet molecular oxygen (O-1(2)) in TiO2 photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H2O2, and ethanol on the amount of O-1(2) produced by photo excitation of P25 TiO2 were measured. The same additives were employed to investigate the effect on the amount of O-center dot(2)-produced. Comparison between the effects on O-1(2) and O-1(2)-suggested that O-1(2) is formed by the electron transfer mechanism, the reduction of molecular oxygens to O-center dot(2)- by photogeneFated electrons and the subsequent oxidation of center dot O-2(-) to O-1(2) by photogenerated holes. The formation of O-1(2) decreased at pH < 5 and pH > 11, indicating that the intermediate O-center dot(2)- is stabilized at the terminal OH site of the TiO2 surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO2 photocatalysts were compared on the formation of O-1(2) and O-1(2)- in an aqueous suspension system. The formation of O-1(2) was increased with decreasing size of TiO2 particles, indicating that a large specific surface area causes a higher possibility of reduction producing O-center dot(2)- and then a large amount of O-1(2) is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of O-1(2).