Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (D-e < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (D-e = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Audelta-CO complexes formed on oxygen vacancies easily lose CO (D-e = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from An to TiO2 with formation of strongly bound Audelta+CO complexes (D-e = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.