The mechanism of superb photocatalytic activity of TiO2 has been studied by synchrotron radiation photoelectron spectroscopy. Photocatalytic activity was evaluated by X-ray photoelectron spectroscopy (XPS), and photogenerated carrier lifetimes were measured by pump–probe time-resolved XPS (TR-XPS). Single-crystal rutile and anatase TiO2 with well-defined surfaces were investigated, and the correlation between photocatalytic activity and the carrier lifetime has been discussed based on the experimental results. The carrier lifetime was determined by measuring the spontaneous change of the electron binding energies, which corresponded to generation and quenching of the surface photovoltage. Photocatalytic activity was evaluated by the rate of decrease in the C 1s XPS peak intensity due to photocatalytic decomposition and desorption of adsorbed acetic acid molecules. There is a linear and positive correlation between the carrier lifetime and photocatalytic activity; the longer the carrier lifetime, the higher the photocatalytic activity. Since the carrier lifetime is sensitive to the surface potential, photocatalytic activity can be tuned by changing the surface potential and controlling the carrier behavior. This was demonstrated by introducing a rutile/anatase boundary on the TiO2 surface.