The effect of water vapor on the SCR of NO by propane over Co-MFI zeolite was investigated with respect to behavior of surface NCO species. In the absence of water vapor, the NO conversion at lower temperatures below 648 K decreased with time-on-stream. In the presence of water vapor, on the other hand, the NO conversion was higher than in the absence of water vapor and the catalyst deactivation was not observed. From the measurement of in situ IR spectra, it was found that NCO species accumulates on the catalyst in the absence of water vapor and then leads to deactivation of catalyst. In the presence of water vapor, however, it was found that the NCO species were easily hydrolyzed to ammonia and carbon dioxide, and therefore, the higher SCR activity was obtained. Thus, NCO species, which are known to be an important intermediate to form nitrogen in HC-SCR, could cause self-poisoning in the absence of water vapor. (C) 2007 Elsevier B.V. All rights reserved.