The interaction of O-2 with reduced rutile TiO2(1 1 0)-(1 x 1) has been studied by means of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and photoelectron spectroscopy (PES). It is found that the interaction of O-2 with TiO2(1 1 0) depends strongly on the reduction state of the TiO2(1 1 0) crystal. High-resolution STM studies revealed that the energy barrier for the non-vacancy-assisted, 2nd O-2 dissociation channel decreases with increasing crystal reduction. Additionally, it is found in the STM studies that the Ti interstitial diffusion is slightly more facile in high-reduced TiO2(1 1 0) crystals compared to low-reduced ones. Accompanying TPD studies revealed that the line shape of the O-2-TPD peak occurring between similar to 360 K and similar to 450 K depends on the crystal reduction state. For high-reduced TiO2(1 1 0) crystals characterized by large terraces most O-2 molecules desorb at similar to 386 K, whereas O-2 desorption is peaking at similar to 410 K for low- and medium-reduced crystals. Furthermore, the O-2-TPD experiments revealed a highly non-linear behavior of the O-2 desorption peak integrals as function of the crystal reduction state. The presented results point to an ionosorption model where the adsorbates withdraw the excess charge (Ti3+) from the near-surface region at temperatures < similar to 360 K and where Ti interstitials react with oxygen species on the surface at temperatures >= similar to 360 K. (C) 2011 Elsevier B. V. All rights reserved.