The electrical conductivity (σ), photoconductivity and photocatalytic reactivity in doped crystalline TiO2 were measured as a function of the oxygen partial pressure (Po2), temperature, doping type and UV irradiation. The Po, dependence of σ suggests that the predominant atomic defects in pure TiO2 are oxygen vacancies (V(o)..) and interstitial titanium ions (Ti(i)...), but the dominant defect is changed with Po2 and temperature. The photoexcited electrons in reduced and/or n-type doped TiO2 enhance both the photoconductivity and the photocatalytic reactivity by the reduction process. Therefore, these behaviors are strongly dependent on the electron concentration.