Oxidation of CO in the absence and in the presence of H-2 was studied on TiO2 and 1% Au/TiO2 catalysts with the help of Fourier transform infrared spectroscopy and mass spectroscopy at 300-373 K. It was found that CO adsorbed on edged and kinked Au atoms are the active surface species for CO oxidation. Oxygen adsorbs on Au particles, too. Activation of hydrogen on the metallic part of the catalyst reduces the extent of CO oxidation. Formation of formaldehyde (both on the surface and in the gas phase) was observed, which also contributes to the decrease of the extent of CO oxidation. It was stated that Au/TiO2 catalyst - although it is very active in CO oxidation in the absence of hydrogen - is not appropriate for the preferential oxidation of CO in CO + O-2 + H-2 gas mixtures.