In this work, the UV-stimulated processes in the TiO2–COg system were explored by in situ FTIR spectroscopy and photoelectron spectroscopy. Kinetic dependences of the total band intensity for adsorbed CO and background absorption, as well as the positions of the Fermi level in TiO2 before, during, and after UV irradiation of the system, were analyzed and compared. The observed reversible shift in the adsorption–desorption equilibrium was explained by changing the electronic state of the photocatalyst under UV irradiation. The proposed mechanism of photostimulated CO adsorption and CO re-adsorption in the TiO2–COg system is described in detail. A deflection of reversibility of molecular photodesorption–re-adsorption dynamics is caused by the side reaction of CO photooxidation and the formation of strongly bound surface carbonates.