The effect of addition of alkali metals was investigated on the catalytic activity of cobalt oxide (Co3O4) for the direct decomposition of NO. Although Co3O4 was totally inactive, NO decomposition over Co3O4 was significantly promoted by the addition of alkali metals (Li, Na, K, Rb and Cs). Potassium was found to be the most effective additive. The addition of alkali metals increased not only the surface area but also the specific activity per unit surface area. This suggests that catalytically active sites are created on the Co3O4 surface as a result of interaction with alkali metals. The interface between alkali metals and Co3O4 is clearly essential in the NO decomposition reaction. Characterization of the catalyst using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and desorption of O-2 (O-2-TPD) revealed that traces of added alkali metals weaken the Co-O bond strength and promote oxygen desorption from Co3O4, resulting in the formation of cobalt species with a slightly lower valence state. We conclude that these effects, caused by addition of alkali metals, are responsible for the activity enhancement of Co3O4. (C) 2003 Elsevier B.V. All rights reserved.