Copyright © 2020 American Chemical Society. Gold nanoparticles of small sizes with a highly uniform dispersion were successfully deposited on several crystalline forms of Ta2O5 (pseudohexagonal, orthorhombic, and pyrochlore) by the sol immobilization method. The pseudohexagonal Ta2O5 (TT-Ta2O5) synthesized by a hydrothermal method showed the highest catalytic activity for CO oxidation as a support for gold catalysts. The temperature at 50% CO conversion was -11 °C for 1 wt % Au/TT-Ta2O5, and the temperature was 23 °C at 100% CO conversion (space velocity of 20000 mL h-1 gcat-1). To investigate the reaction mechanism, in situ diffuse reflectance infrared Fourier transform spectroscopy for Au/TT-Ta2O5 was performed, and the sequential delivery of adsorbed CO on Ta2O5 sites and Au sites to the active sites was observed at -120 °C. The results of temperature-programmed CO reduction suggested that the oxygen adsorbed at -100 °C contributed mainly to the high catalytic activity of CO oxidation, while the lattice oxygen played a minor role for CO oxidation from -60 to 100 °C, which was the same temperature range with the actual reaction condition.