NO decomposition over supported alkaline earth metal oxide catalysts was strongly dependent on the type of metal oxide support. Y₂O₃ was the most effective support. Ba/Y₂O₃ showed the highest NO decomposition activity, which decreased in the order of Ba/Y₂O₃ > Sr/Y₂O₃ > Ca/Y₂O₃ > Mg/Y₂O₃ ~ Y₂O₃. The catalytic activity of Ba/Y₂O₃ for NO decomposition at 900°C gradually increased with reaction time. The activity enhancement was due to the decomposition of barium carbonate into barium oxide during the reaction. Barium carbonate was completely decomposed by reduction with H₂ at 900°C, resulting in significant enhancement of NO decomposition activity. The activity of Ba/Y₂O₃ increased with higher barium loading up to 5 wt%, and then became constant. There was a strong relationship between NO conversion and the amount of NO adsorption on Ba/Y₂O₃, suggesting that NO adsorption sites are the reaction sites for NO decomposition. The relationship between the activity for 1-butene isomerization, which is an indicator for the basicity of Ba/Y₂O₃ catalysts, and the activity for NO decomposition suggests direct participation of basic sites in the NO decomposition reaction over supported alkaline earth metal oxide catalysts.