We report results of a comprehensive study on the effect of bulk defects on the catalytic behavior of Au/TiO2 catalysts in the CO oxidation reaction, combining quantitative information on the amount of surface and bulk defects from in situ non-contact electrical conductivity measurements after pretreatment and during reaction with information on the electronic/chemical state of the Au nanoparticles (NPs) provided by in situ IR spectroscopy. Treating the catalyst in strongly reducing atmosphere (10% CO/90% N-2) at 400 degrees C results in a distinct increase in electrical conductivity, indicative of the formation of defects (oxygen vacancies), which are stable at 80 degrees C in N-2. Long-term kinetic measurements performed at 80 degrees C show a distinctly lower activity of the bulk reduced catalyst, which increases slowly with time on stream, directly correlated with the decreasing abundance of bulk defects. The detrimental effect of bulk defects on the CO oxidation activity is shown to originate from the lowered CO adsorption strength and hence very low COad coverage on the Au NPs due to electronic metal-support interactions (EMSIs) induced by the presence of TiO2 bulk defects, in good agreement with our recent proposal (Wang et al., ACS Catal. 7 (2017) 2339). For reaction at -20 degrees C, EMSIs lead to a promoting effect on the CO oxidation, pointing to a change in the dominant reaction mechanism, away from the Au-assisted Mars-van Krevelen mechanism dominant at 80 degrees C. The role of EMSIs in the CO oxidation reaction and its temperature dependence is discussed in detail. (C) 2017 Elsevier Inc. All rights reserved.