Ag–Al₂O₃ catalyst prepared by the sol–gel method was studied for the selective catalytic reduction (SCR) of NO by various hydrocarbons (HC-SCR). The rates of NO reduction and hydrocarbon conversion depend strongly on the type of hydrocarbon reductant; the rates were higher for the higher normal alkanes, higher for alkene than alkane, and higher for normal alkane than branched alkane. The formation and reaction of hydrocarbon-derived species adsorbed on the surface were investigated by in situ FTIR spectroscopy. The apparent rate of acetate formation in the hydrocarbon + O₂ reaction and the rate of nitrate reaction in hydrocarbon + O₂ also depend on the nature of the hydrocarbon with a similar activity pattern to that for the SCR reaction. For the various hydrocarbons used, the rate of nitrate reaction in hydrocarbon + O₂ was close to the steady-state rate of NO reduction, indicating that the nitrate is a possible intermediate in this reaction. The reaction of nitrates with oxygenated hydrocarbon species, possibly the acetate, was proposed to be a crucial step in HC-SCR. A proposed mechanism explains the hydrocarbon effect on the de-NO_x activity: the NO reduction activity depends on the reactivity of the hydrocarbon molecule, which affects the rate of hydrocarbon oxidation to oxygenates and hence the rate of nitrate reaction with oxygenated hydrocarbons.