NO reduction with propylene over Co/Al2O3 and Co-Sn/Al2O3 catalysts has been investigated. For the Co/Al2O3 catalyst, a calcination temperature exceeding 800 degreesC led to a decrease of NO conversion. Calcination of the Co/Al2O3 catalyst at 1000 degreesC resulted in the formation of alpha -Al2O3 and Co3O4. The presence of 20% water vapor showed a significant shift for the maximum NO reduction temperature from 450 to 600 degreesC over Co/Al2O3. It has been found that modification of 6 wt% Co/Al2O3 with 2 wt% Sn significantly enhanced the catalyst thermal stability and improved the inhibitory effect of water on NO conversion and reaction temperature. The promotional effect of Sn on the catalyst thermal stability was attributed to the suppression of the phase transformation from highly dispersed Co2+ species on gamma -Al2O3 to alpha -Al2O3 and Co3O4. The smaller influence of water vapor on NO reduction conversion and temperature over Co-Sn/Al2O3, compared to Co/Al2O3, was attributed to the dispersion effect of Sn species on Co2+ species as well as the involvement of Sn species in NO reduction at a relatively lower temperature. The synergetic effect between the octahedral Co2+ species and gamma -alumina plays a significant role in the catalysis of NO selective reduction by C3H6.