The mechanistic cause of the promoting effect of CeO2 on the activity of SnO2/Al2O3 catalyst for the SCR of NO (x) by propene was investigated using X-ray photoelectron spectra (XPS) and in situ Fourier transform infrared (FT-IR) spectroscopy. FT-IR measurements have revealed that the role of CeO2 on the CeO2-SnO2/Al2O3 catalyst is to contribute to the formation of formate, acetate and nitrate species, and to promote the reaction between nitrates and hydrocarbon-derived species to form isocyanate (-NCO), which is a reaction intermediate for NO (x) reduction.