Bromine and chlorine co-doped nanocrystalline titanium dioxide are synthesized by a hydrothermal method using titanium chloride in a mixed hydrobromic acid−ethanol system. TiO2 with anatase, mixed anatase/rutile, rutile, mixed rutile/brookite, mixed anatase/rutile/brookite, and rutile phases are obtained, in that order, by increasing the acidity through addition of HBr. Br- and Cl- co-doping causes the absorption edge of TiO2 to shift to a lower energy region. The photocatalytic activity of doped TiO2 with mixed anatase/rutile phases exceeds that of commercial TiO2 photocatalyst Degussa P-25 for water splitting into H2 and O2 under ultraviolet light.