Temperature-programmed reduction (TPR), synchrotron-based time-resolved X-ray diffraction (TR-XRD), extended X-ray absorption fine-structure (EXAFS), and X-ray absorption near-edge structure (XANES) were b employed in this work to systematically study the reaction of CuO and CU2O With CO gas molecules. Both TPR and isothermal reduction results showed that CuO was easier to reduce than CU2O under the same reaction conditions. In situ measurements of XRD and XANES showed a direct transformation pathway for CuO reduction (CuO --> Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO --> Cu2O --> CU) with a limited supply of CO. An induction period for CuO reduction was seen and increased with decreasing temperature. 0 vacancies in CuO were observed during the induction period that could foster formation of a nonstoichiometric metastable copper-oxide species. The metastable species can either react rapidly with CO to form metallic copper during high CO flow rates or can relax into a CuO lattice when the supply of CO is limited. The CU2O-like intermediate showed extra disordered oxygen in the empty tetrahedral sites Of CU2O. Based on these observations, a possible mechanism for CuO reduction is proposed. This study demonstrates that the mechanism for the reduction of an oxide by CO can vary considerably with the experimental conditions (gas flow rate, temperature, sample size, etc.), and its complex kinetics cannot be described by a single n(th)-order expression over the entire range of reaction. The behavior of CuO-based catalysts is discussed in the light of these results.