In-situ analyses on reactive sputtering processes used to deposit photocatalytic TiO2 were carried out using a quadrupole mass spectrometer combined with an energy analyzer. High-energy negative oxygen ions (O-) accelerated by the cathode sheath electric field of several hundred volts and fragments sputtered from the target were analyzed in relation to the oxygen flow ratio and total gas pressure (P tot). With increasing the oxygen flow ratio over 15%, the deposition rate decreased markedly where the target surface was fully oxidized confirmed by in-vacuo X-ray photoelectron spectroscopy analysis on the target surface. High-energy O- ions with a kinetic energy of several hundred eV corresponding quantitatively to the full cathode voltage were distinctly observed. Such high-energy O- species dominant at P tot = 1.0 Pa showed a marked decrease and disappeared at 3.0 Pa. The photocatalytic activity of the anatase TiO2 films deposited at 3.0 Pa was much higher than that of the film deposited at 1.0 Pa, which could be attributed to the presence of many more recombination centers introduced by the bombardment of high-energy O- ions.