This work determined the local concentration of electrons in the grain boundary layer of TiO2 (rutile) at elevated temperatures (1223-1323K) in the gas phase of controlled oxygen activity (10(-11) Pa < p(O-2) < 10(-5) Pa). The determination is based on the measurements of thermoelectric power for both single crystal and polycrystalline specimen of pure TiO2. It is shown that the concentration of electrons in the grain boundary layer of polycrystalline TiO2 is larger than that in the bulk phase by a factor ranging between 4 at 1323 K, and 13 at 1273 K. The present finding is crucial in understanding the reactivity of polycrystalline TiO2 and its performance in solar energy conversion by TiO2. (C) 2013 Published by Elsevier B.V.