The surface species occurring on Ag/Al2O3 in SCR of NO by propene were characterized using temperature programmed techniques. An experimental set of three steps containing the step of production of the SCR species, the temperature programmed desorption (TPD) step and the temperature programmed oxidation (TPO) step shows that there are at least five groups of surface species oil Ag/Al2O3. These species consist of two main types. The weak adsorbed species (the TPD species) decompose during TPD step while the strong adsorbed species (the TPO species) are necessarily oxidized with some oxidants in TPO step. The reactivity of all the surface species with NO + O-2 was also studied to identify which species is an intermediate. The reactivity results suggest that some of the TPO species is more important to the reduction of NO to produce N-2. Some of the TPD species is an inhibiter for C3H6-SCR of NO on low loading Ag/Al2O3 but become an assisting intermediate on high-loading catalyst. (C) 2004 Elsevier B.V. All rights reserved.