The influence of water on the dispersion of silica SBA-15 supported vanadia model catalysts has been studied using X-ray photoelectron spectroscopy (XPS) as well as UV-VIS and Raman spectroscopy. XPS characterization of the V 2P(3/2) state reveals the presence of two V5+ states, which are assigned to vanadia with distinctly different cluster size. Upon dehydration a dramatic chan e in their intensity ratio is observed as a result of a substantial increase in the vanadia dispersion. It is shown that the observed changes in vanadia dispersion are directly associated with the changes in the molecular structure of the surface vanadia species. (c) 2006 Elsevier B.V. All rights reserved.