Previous studies have proposed bandgap narrowing by anion (e.g. N, C, or S) or cation doping into the lattice of TiO2. Sulfur is of particular interest, as TiS2 has a close-to-zero bandgap. By mixing TiO2 and TiS2 into oxysulfide, a new semiconductor with a suitable bandgap for photovoltaics or solar water splitting may emerge. In this work, we investigate sulfurization of TiO2 thin films through H2S gas-phase diffusion. The TiO2 films were prepared by the sol-gel method. Transmittance and X-ray diffraction analyses indicate that the TiO2 films have an anatase phase with a direct bandgap of 3.2 eV. The films are sulfurized in H2S under different pressure, temperature and time. It is found that sulfurization results in a second, narrower bandgap phase in TiO2. At 600˚C, the anatase TiO2 almost completely disappears. The results suggest that sulfurization by H2S is effective in narrowing the bandgap of TiO2.