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Conductive Ti-oxide thin films are produced using a reactive sputtering and post-annealing process. The lowest resistivity of Ti-oxide thin films (2.30 × 10–2 Ω-cm) can be achieved after annealing for 1 h at 400 °C in ambient O2. Additionally, the Hall measurement results indicate that the carrier concentration increases during the initial 1-h annealing process before decreasing during subsequent annealing. By curve fitting the Ols core-level peaks in the x ray photoelectron spectroscopy (XPS) spectrum of the annealed Ti-oxide thin films, we found that the oxygen (O) vacancy concentration monotonically increases with annealing time, which differs from the behavior of the carrier concentration regarding annealing time. This means that the O-vacancy mechanism alone cannot explain the formation of carriers in Ti-oxide thin films. By curve-fitting core-level Ti peaks in the XPS spectrum of annealed Ti-oxide thin films, a Ti3+-to-Ti4+ substitution reaction in the TiO2 phase of the Ti-oxide thin film after annealing plays the dominant role in the formation of conduction carriers. Instead of the O-vacancy mechanism, the Ti3+-to-Ti4+ substitution mechanism can explain the concentration of carriers in Ti-oxide thin films following annealing.