Optical absorption spectra and electrical resistivity measurements are given of ruthenium-doped TiO,. The effect on the system of doping with shallow donors (Ta, Nb) and of reducing in vacuum and in hydrogen is investigated. The results are discussed in view of recent EPR, Mossbauer, and thermogravimetric measurements. Fully oxidised Ru-doped TiO, shows an absorption band which is attributed to acceptor transitions of the Ru4+ ion. Absorption bands are observed in reduced and donor-doped TiO, which can be assigned to donor transitions of Ru3+ and Ru2+ ions. It is deduced that both treatments lead to reduction of Ru4+ to Ru3+ and Ru2+ before the appearance of free carriers, although differences exist between the effect of doping, of reducing in vacuum, and of reducing in hydrogen. An energy level scheme for the different valence states of Ru in TiO, is proposed.

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