Gas transient methods have been used to probe reaction pathways in catalytic CO oxidation under operando conditions. A low dead volume reactor is described, which permits monitoring of surface reaction species by repetitive FTIR while using mass spectrometry to continuously monitor reaction products and conversion throughout transient gas switching. CO desorption rates are measured from an active Au/TiO2 catalyst under He and CO2 and are consistent with literature values for CO desorption from An single crystals and Au films on TiO2. The rate of the reactive removal of adsorbed surface CO(a) was used to determine a turnover frequency of about 0.043 s(-1) for CO oxidation on the Au particles, comparable to the rate obtained in a steady state flow reactor. Gas switching is used to investigate the storage of oxygen on the catalyst. It is found that following O-2 exposure, the amount of adsorbed active oxygen is either undetectably small, less than 3.1 mu moles O/g(catal), or it desorbs very rapidly at 298 K. Although storage of oxygen is very ineffective on this catalyst, CO2 is stored by its dynamic interaction with the support to form carbonates or bicarbonates causing slow removal of product CO2 from the reactor bed. These results are summarized by a scheme describing the pathways for CO oxidation. (c) 2006 Elsevier B.V. All rights reserved.