The adsorption of CO on TiO2 supported gold has been investigated both under a constant pressure static system and under a closed recirculation system with liquid nitrogen cooled trap. Au/TiO2 with 3.3 wt% loading and 3.5 nm mean particle diameter of Au was prepared by deposition-precipitation. Adsorption of CO on Au/TiO2 was 90% reversible and satisfied the Langmuir isotherm. The amount of CO2 produced during CO adsorption agreed well with the amount of irreversible CO adsorption. TiO2 powder produced only 1/185th the amount of CO2 at 273 K compared to gold powder, even though the amount of CO adsorbed per unit surface area was similar to that of gold powder. The results indicate that a reaction between oxygen adsorbed on the surface of small gold particles with CO is one of the major reaction pathways in forming CO2.