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Adsorption of electron acceptors on partially reduced TiO2 leads to the localization of mobile conduction band electrons which is indicated by the appearance of an ESR signal with g = 2.003. The localization process is accompanied by an electron transfer from donor centers in reduced TiO2 (Ti3+ ions) to adsorbed molecules. The ESR spectra show a decrease of the Ti3+ signal intensity at gr -1.96. Additionally, in the case of O2, SO2 and C6H5NO2 adsorption, the ESR signals of the respective anion radicals are observed. Illumination of samples with an electron acceptor adsorbed enhances the g - 2.003 signal. The activation energy corresponding to the temperature dependence of the localization process has been determined to be ~ 1 kcal/mole in the temperature range - 140 to - 40 °C. In the case of O2 and N2O the signal at g = 2.003 could be annihilated chemically by using propylene which reacted with the adsorbed electron acceptor and thus removed the species which reduced the electron mobility

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