Catalytic reduction of NO by propene in the presence of oxygen was studied over Sn-Al2O3 prepared by the sol-gel method. The maximum NO conversion did not change much with Sri loading (1-5 wt%), although the effective temperature window for NO reduction shifted to lower temperatures with increasing Sri loading. X-ray diffraction (XRD), and X-ray absorption spectroscopy (XANES and EXAFS) revealed the formation of finely-divided SnO2 crystallites in Sn-Al2O3. It was deduced that not only the surface vicinity of supported SnO2, which can be reduced at lower temperatures, but also alumina participate in the reaction as catalytically active sites. The catalytic performance of Sn-Al2O3 was markedly improved by an H2O treatment at 873 K. The H2O treatment did not affect the crystal structure of the catalysts and the dispersion state of supported SnO2, but promoted the removal of Cl- ions, which originated from the SnCl4 precursor. The activity enhancement by the H2O treatment was accounted for by an increase in the surface SnO2 concentration as active sites and by the modification of oxo-tin species as active sites. Reaction kinetic studies suggested that NO reduction on the fresh and the H2O-treated catalysts proceeds through the same reaction pathway in which the surface NOx adspecies and C3H6-derived species play an important role for NO reduction.