Silica-supported vanadium (1-8 wt%) and vanadium (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy, NO + NH3 rectangular pulses and oxygen chemisorption. The presence of different vanadium species was correlated with activity and selectivity during the methane partial oxidation reaction. The pre-impregnation of the silica support with sodium favors vanadium dispersion, but strongly diminishes V=O concentration due to the formation of orthovanadate-like compounds. As a result of these modifications, methane conversion is strongly inhibited while formaldehyde decomposition is favored.