The catalytic decomposition of NO over cobalt oxide was significantly enhanced by the addition of alkali promoters. The promotional effect was the highest with Na and the optimal Na/Co atomic ratio was 0.015. The reaction rate increased with increasing temperature up to 650 degrees C. At 550 degrees C, the rate measured at low conversions for the most active catalyst was 0.28 mu mol N-2/g s. In contrast, very low decomposition rate was observed when Na was added to ZrO2. This suggests that both Na+ and Co3O4 are necessary for the high NO decomposition activity. In (NO)-N-15 temperature programmed desorption, the presence of Na in the sample resulted in the appearance of desorption peaks of NO, O-2 and N-2 at temperatures higher than 450 degrees C. X-ray diffraction of a post-reaction sample detected a NaNO3 phase. It was proposed that the presence of Na facilitated the desorption of oxygen from Co3+ to form Co2+ via the formation and decomposition of NaNO3, and Co2+ was the active site for NO decomposition.