The molecular structure and reactivity of supported solid V2O5/SiO2 catalysts with 6.5 wt% V content and surface densities in the range 0.97-2.04 V atoms/nm(2) have been studied by in situ Raman spectroscopy and activity measurements for the oxidation Of SO2. Different sol-gel routes. resulting in variations of surface area and pore volume, were used for preparing the catalysts. These catalysts were found to be practically inactive for the oxidation of SO2 while their molecular structure evolves from isolated distorted tetrahedral O=V-(O-Si)(3) units exhibiting the characteristic Raman band due to V=O terminal stretch at 1032-1035 cm(-1) to bulk V2O5 crystals when vanadia loading exceeds the maximum coverage of surface vanadium oxide species on silica. Promotion of the catalysts with cesium (by impregnation with Cs2SO4 at Cs:V = 3) results in thorough structural transformation of the studied materials which is followed by dramatic improvements in their reactivity for SO) oxidation. Amorphous and crystalline vanadia is extracted during calcination and dissolved in a sulfate molten salt, which remains distributed at the surface of the silica support. In situ Raman spectroscopy shows that vanadium occurs in the sulfate molten salt predominantly in the form of the mononuclear (VO2)-O-V(SO4)(2)(3-) molten oxosulfato complex (with characteristic bands at 1034 cm(-1) due to v(V=O) and 942 cm(-1) due to sulfate). A fairly good correlation between the surface areas Of V2O5/SiO2 precursors and TOF (turnover frequency) values was found for the V2O5-Cs2SO4/SiO2 molten salt catalysts. Activation of these catalysts, following exposure to a SO2/O-2/N-2 mixture, results in uptake of SO3 and formation of a pyrosulfate molten salt, in which-as shown by the in situ Raman spectra-vanadium occurs predominantly in the form of the binuclear ((VO)-O-V)(2)O(SO4)(4)(4-) molten oxosulfato complex (with characteristic bands at 1046 cm(-1) due to nu(V=O), 830 cm(-1) due to nu(S-O-V) and 770 cm(-1) due to V-O-V). Exposure of the catalysts to reducing atmosphere, i.e., SO2/N-2. results in formation of V-IV species, which are catalytically inactive for the oxidation Of SO2. The results should be very useful for an understanding of the structure/activity relationships in V2O5-Cs2SO4/SiO2 molten salt catalysts. (C) 2004 Elsevier Inc. All rights reserved.