In situ Raman spectroscopy has been used at temperatures LIP to 400 degrees C under O-2. NH3/N-2, H-2/N-2), NH3/NO/O-2/N-2, O-2/H2O/N-2), and SO2/O-2/N-2 for Studying tile influence of these gases oil the molecular Structure Of V2O5/TiO2 catalysts with V surface density, it, in the range 2.5-18.7 VOx/nm(2). The catalyst activities for the SCR of NO by NH3 have been determined to derive structure-activity relationships in combination with the Raman data. Isolated monovanadates and polyvanadates are formed at various proportions (depending oil the loading) oil the catalyst Surface Under dehydrated conditions. The hand positions and characteristics are discussed in terms of possible configurations for the dispersed VOx, species. The bands observed, the surface composition, and the bond conservation rule allow to propose a small size for the V-O-V chains of polyvanadates (i.e., 2, 3). The reducing action of NH3 is favored in the presence of adjacent V sites; at low loadings. the presence of NH3 has not effect on the structural properties of surface VOx. The reducibility in H-2 follows an oppostite trend and is favored at low n(s), as indicated by both effect oil the structural properties of surface VOx. The reducibility in H-2 follows all opposite trend and is favored at low it, ill Situ Raman and H-2-TPR. The SO,) presence affects only the molecular Structure of catalysts with low n(s). for which a significant part of surface TiO2,) sites are vacant; the effect (judged from the ill Situ Raman data) is merely one of driving the dispersed vanadia species in a state of"virtually" high Surface density by crowding them together, thereby providing more adjacent V sites for activation of NH3 in SCR reaction conditions. The NO TOF Values initially increase with increasing its, suggesting that the number of active sites per V atom increases with increasing its below s or at conditions of "virtually" high n(s) (in the presence of monolayer. The formation of adjacent V-O-Ti sites is favored either at increasing n(s), or at conditions of "virtually" high n(s) (in the presence of SO2). The increase in the number of such centers per V atom correlates with the increase in TOF values with increasing its. (c) 2006 Elsevier Inc. All rights reserved.