NO reduction by CO reaction was comparatively studied over microwave plasma pretreated CuO/TiO2 catalysts employing transmission electron microscopy (TEM), hydrogen temperature programmed reduction (H-2-TPR), and in situ Fourier- transform infrared spectroscopy (in situ FTIR). The catalytic performances indicated that a remarkable improvement in activity and selectivity was achieved after microwave plasma pretreatment, which was dependent on the microwave plasma pretreatment time. The results also suggested that a high active oxygen species (O*) was formed on the surface of plasma pretreated CuO/TiO2 catalysts, which led to the easy oxidation of NO to NO2 at low temperature even without introducing any additional O-2 gas into the reaction system. Therefore, these high active oxygen species (O*) should play an important role for the exaltation of the catalytic performances of the catalysts. (C) 2011 Elsevier B.V. All rights reserved.