The deactivation of noble metal catalysts by SO2 and H2O is a common issue in the post combustion treatment of flue gases from sintering processes in the steel industry. In an effort to develop SO2-tolerant CO-oxidation catalysts, herein, we investigated the effect of SO2 and H2O on the catalytic activity of Pt/TiO2(P-25) catalysts for CO oxidation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and diffuse-reflectance infrared Fourier-transform (DRIFT) spectroscopy. Pt/TiO2(P-25) catalysts, in the absence of SO2 and presence of H2O, enhanced the activity and stability of CO oxidation, while being largely suppressed and irreversibly deactivated in the presences of SO2. The XPS and TEM results suggested that variations in the Pt particle size and oxidation state were not major causes of the deactivation. Instead, according to DRIFT spectra, the interaction between CO and H2O at the metal-support interface was weakened after the formation of TiOSO4 on the TiO2 surface in the presence of SO2. This resulted in a loss of the previously observed enhancement of CO oxidation under humid conditions. These results indicate that in the presence of SO2, the formation of TiOSO4 is the major cause of irreversible deactivation. Therefore, removal of the TiOSO4 layer from the TiO2 surface is a crucial step for catalyst regeneration.