Transient studies on the oxidation of CO over Au/TiO2 have been performed at 243-363 K using a time-resolved fast-scanning Fourier transform infrared (FTIR) spectrometer coupled with a quadrupole mass spectrometer. Different CO-O-2 interactions were performed by stepping CO into a steady-state He flow, O-2 into a steady-state CO flow, CO into a steady-state O-2 flow, and CO/O-2 into an He flow. The CO oxidation reaction occurs at all temperature ranges investigated whether CO is preadsorbed or O-2 is preadsorbed, indicating that there is no competition between CO and O-2 for adsorption at a temperature range of 243-363 K (noncompetitive adsorption). It suggests that CO and O-2 adsorption occurs on two different types of sites. When there is no O-2 present in step feed (i.e., step CO into He), CO immediately builds up on the surface (rapid adsorption of CO) and desorbs slowly after cutting off CO flow. However, when O-2 is present in the step feed (i.e., step CO/O-2 into He), adsorbed CO reacts with adsorbed oxygen immediately (fast surface reaction), whereas CO2 product desorption monitored by mass spectroscopy appears to be very slow. Slow desorption Of CO2 product is claimed to be the rate-limiting step of CO oxidation over Au/TiO2 at a temperature range of 243-273 K. (c) 2005 Elsevier Inc. All rights reserved.