The interaction mechanism between SO2 and NO on gamma-Al2O3 was explored by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and outlet response of the concentrations of NO, NO2 and SO2 under exposure of Al2O3 to SO2 and/or NO in the absence or presence of oxygen at 150 degrees C. The results showed that SO2 promoted NO oxidation and NO transformed weakly adsorbed SO2 into strongly adsorbed species on y-Al2O3, and the presence of O-2 facilitated this transformation. An interaction mechanism between SO2 and NO on -y-Al2O3 was thus postulated. The exposure of Al2O3 to SO2 and NO in the presence of O-2 resulted in the formation of at least two types of intermediates. One type was [SO3NO], which decomposed to form NO2, and the other type was [SO3NO2], which decomposed to form SO3. The decomposition of both intermediates probably formed O vacancies replaceable by gaseous O-2. Crown Copyright (C) 2011 Published by Elsevier BM. All rights reserved.