New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. It? situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)(2)VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 degrees C under vacuum, leading to a site living a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state: (v(4+)-O-)(*) reversible arrow (v(5+)=O2-). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed. (c) 2007 Elsevier B.V. All rights reserved.