Silica-supported molybdenum(1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%)/SiO2 surface were MoO3 and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na2Mo2O7 species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support.