Titanium oxide thin scales isothermally grown at 850 C on titanium metal in pure oxygen, pure water vapor, or water vapor/oxygen mixtures were submitted to room temperature photoelectrochemical characterization (PEC) to assess the nature of oxides in function of the composition of the oxidizing atmosphere. The results indicated that minor lower valency oxides were present in addition to TiO2 rutile, of which the bandgap energy was experimentally determined to be 2.95 eV. As bandgap energies of Ti3O5 and Ti2O3 have not been reported in the literature, an empirical model relating bandgap energy and reticular enthalpy was revisited and adapted to the case of oxides. From photoelectrochemical measurements on thermal oxides, new bandgap values of 3.28 eV for Ti2O3 and 4.04 eV for Ti3O5 were obtained, correctly matching the empirical model. High sensitivity of the technique was demonstrated as these phases could be identified by PEC on samples grown in oxygen, where classical techniques of characterization like X-ray diffraction could not detect them. They were observed in larger amounts on samples grown in the presence of water vapor, showing closer metal-oxide equilibrium during thermal oxidation. (c) 2007 The Electrochemical Society.