The present work reports the thermoelectric power of high- purity single-crystal TiO2 in the temperature range 1073-1323 K and in gas phases of controlled oxygen activities, p(O-2), in the range 10(-13) to 7.5 x 10(4) Pa. The thermoelectric power versus log p(O-2) dependence for strongly reduced TiO2 at p(O-2) < 10(-5) Pa may be approximated by a slope of 1/ 6, which is consistent with the defect disorder governed by electronic charge compensation of oxygen vacancies. The thermoelectric power data confirm that oxygen vacancies are the predominant ionic defects. These data indicate that TiO2 at high p(O-2) exhibits p-type properties. It is shown that the p(O-2) related to the n-p transition increases with increase of temperature.