The lean NOx performance and catalytic properties of In2O3/Al2O3 catalysts were investigated. High lean NO,, activity was observed when propene was used as a reductant in the presence of 9% O-2 and 7% H2O at a space velocity of 30,000 h(-1). The optimum lean NOx activity of In2O3/Al2O3 catalysts was observed at a loading of 2.5 wt% indium on y-Al2O3 which was prepared by a Sol-gel technique (230 m(2)/g). When propane was used as a reductant, the In2O3/Al2O3 catalyst did not promote NOx reduction compared to the alumina substrate. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed reduction (TPR) have been used to characterize a series of In2O3/Al2O3 catalysts to increase understanding of the surface structure of indium oxide species on the alumina support. The XRD data indicated that crystalline In2O3 was present at In2O3 loadings >5 wt% and the quantity of the crystalline phase increased as a function of indium loading. XPS results suggested that indium oxide existed as a well-dispersed phase up to 10 wt% indium. The well-dispersed or reducible indium oxide species below 400 C in TPR experiments were assigned as the sites which activate propene to oxygenated hydrocarbons such as acetaldehyde and acrolein. Alumina sites readily utilize the oxygenated hydrocarbons to reduce NOx. A bifunction mechanism was proposed to explain high NO, reduction over In2O3/Al2O3 catalysts.