The role of the Al2O3 support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated. A series of Ag/Al2O3 catalysts were prepared by impregnation method and characterized by N-2 pore size distribution, XRD, UV-Vis, in-situ FT-IR and acidity measurement by NH3 and pyridine adsorption. The catalytic activity differences of Ag/Al2O3 are correlated with different properties of Al2O3 supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al2O3 (Ag/Al2O3 (SG), showed higher NO, conversion (65% at 400 degrees C, compared with the respective catalysts made from commercial Al2O3 (Ag/Al2O3 (GB), Ag/Al2O3 (ALO), (similar to 26 and 7% at 400 degrees C). The higher surface area, acidity and pore size distribution in sol-gel prepared Al2O3 (SG) results in higher NO and hydrocarbon conversion. Based on the UV-vis characterization, the activity of NO reduction is correlated to the presence of Ag-n(delta+) clusters and acidity of Al2O3 support was found to be one of the important parameter in promoting the formation and stabilization of Ag-n(delta+) clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al2O3 (SG) is confirmed, which could also contribute to low temperature hydrocarbon activation and improve NO conversion. In situ FT-IR measurements revealed the higher rate of -CN and -NCO intermediate species formation over 4wt% Ag/Al2O3 (SG). We conclude that the physico-chemical properties of Al2O3 play a crucial role in NO, conversion over Ag/Al2O3 catalysts. Thus, the activity of the Ag/ Al2O3 catalyst can be tailored by using a proper type of Al2O3 support.