After exposing a reduced TiO2(1 1 0) surface to an O-2 gas, the defect state seen in the band gap for the reduced surface almost disappeared, while O adatoms (O-ad) appeared on the 5-fold Ti rows. We then dosed a CO gas onto the TiO2(1 1 0) surface oxidized by O-18(2) exposure and measured the defect state intensity and the amount of O-18 by photoemission and ion scattering spectroscopy, respectively. With increasing CO dose, the defect state intensity increases, whereas the amount of O-18 decreases. The results obtained clearly show the reaction of CO with Oad probably to form CO2. (C) 2011 Elsevier B. V. All rights reserved.