The CO oxidation with O on an active catalyst has been studied using a TAP (temporal analysis 2 Au/Ti(OH)4* of products) transient technique to gain insights into the CO oxidation mechanism. Taking advantage of the TAP system, we have tried to elucidate the controversial mechanism proposed for the CO oxidation on supported gold catalysts. Pump-probe experiments have been performed to determine the lifetime of surface reactive intermediates involved in the oxidation reaction. In a typical pump-probe experiment the catalyst is first charged with a reactant pulse from the pump valve and then interrogated with a different pulse from the probe valve. Varying the time between the pump and probe pulses gives information related to the lifetime of surface intermediates. The pump-probe experiments together with single-pulse experiments revealed that molecularly and irreversibly adsorbed oxygen contributed to the catalytic CO oxidation. When CO was a pump molecule, the CO yield was not influenced by the pump-probe time interval. This means that CO 2 reversibly adsorbs on the surface with a lifetime long enough to react with oxygen. Moreover, the nature and role of oxygen species in the reaction have been examined in the TAP reactor using 18O transient 2 experiments. The results revealed that oxygen passed over the catalyst without isotope scrambling and that the lattice oxygen atoms exchanged only with CO formed by the CO oxidation reaction.