Transient experiments involving the saturation of the catalyst either with CO or O2, followed by admission of O2 or CO, were performed. Exposure of the activated catalysts to 10 mbar of CO at -60°C led to the formation of CO adsorbed species over only 13% of the gold. Admission of 25 mbar of O2 to the CO-saturated catalyst induced drastic changes on the nature and population of surface species. CO2 was readily formed at the expense of the highly active CO-Au species (2091/cm), at a turnover frequency (TOF) of 1.4 mole/mole Au-min. CO2 production was spread over a longer time scale, with a maximum TOF of 1 mole/mole Au-min. Whether the catalyst is initially saturated with CO or O2 before admitting the other reactant did not affect the total amount of CO2 generated. Transformation of CO into CO2 entailed a hydroxocarbonyl intermediate characterized by an absorption band at 1240/cm. Its rate of disappearing correlated with the rate of formation of CO2. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 1/04/2005).