The effect of alkali metal additives on both the structure of MoO3/SiO2 and its behavior as a catalyst for selective oxidation of methane and methanol to formaldehyde was investigated. The structure of the silica-supported molybdenum oxide catalysts was determined by in situ Raman spectroscopy and the amount of reducible oxygen in the catalyst was determined by temperature-programmed reduction. In the absence of alkali metal, only an isolated surface molybdenum oxide species was present on the silica support. Addition of alkali metals decreased the number of isolated surface molybdenum oxide species and formed new alkali-molybdate compounds due to the weak interaction of the Mo species with the silica surface. The oxygen associated with the alkali-molybdate compounds was generally not available for oxidation reactions. Consequently, the addition of alkali metal decreased the catalytic activity for the oxidation of both methane and methanol. The activity for methane oxidation was found to correlate with the number of remaining isolated Mo species and the activity for methanol oxidation was found to correlate with the amount of reducible oxygen present in the catalyst.