Mesoporous VOx/SiO2 catalysts prepared with a novel method appear to be more efficient for partial oxidation of methane to formaldehyde. In this study, vanadium species present in these catalysts were characterized by TPR measurements, in situ IR and Raman spectroscopies after various high-temperature treatments. The molecular structure of vanadium species appeared to be dynamic with a reversible strong evolution between the hydrated state in ambient air and the dehydrated and dispersed one. Raman and IR spectroscopies allowed identification of the presence of hydroxylated monomeric species under dehydrated conditions. The novel preparation method favored the formation of a greater number of D2 defects compared with a reference V/MCM41 catalyst. In addition, quantitative IR measurements have shown that a higher proportion of these monomeric hydroxylated species was obtained with this method, leading to the conclusion that the hydrolyzed D2 silica defects could be their preferential anchoring sites and that these species would be the most efficient methane oxidation sites, explaining the better catalytic activities of the described catalysts. The positive effect of a water co-feed on catalytic performance was explained by an increase in the number of hydroxylated monomeric species present under steam at the reaction temperature. A structure for the monomeric hydroxylated species is proposed and discussed. (c) 2007 Elsevier Inc. All rights reserved.