The defect structures of photocatalytic improved Degussa P25 powder were evaluated by X-ray photoelectron spectroscopy (XPS), paramagnetic resonance spectroscopy (EPR) and in situ studies of the electrical resistivity in order to understand the charge carrier transport inside the material. Annealing the titanium dioxide in vacuum at temperatures below the phase transition temperature did not increase the oxygen defect concentration drastically. However, annealing Degussa P25 powder in vacuum resulted in a redistribution of Ti3+-lattice states and lattice oxygen vacancies. The diffusion of these lattice oxygen vacancies from the bulk to the surface of the material forms charge carrier trapping sites. Such transformation of the defect structure of TiO2 leads to switching from an insulating state to a conducting state.