Partial oxidation of methane into oxygenates, such as methanol and formaldehyde, over MoO₃/SiO₂ catalyst was studied at 873 K in excess amounts of water vapor. The selectivity of oxygenates was found to increase as the fraction of water vapor increased. This was ascribed to the formation of silicomolybdic acids (SMA) on the catalyst surface during the reaction: SiO₂ + 12 MoO₃ + 2H₂O→H₄SiMo₁₂O₄₀. Then, SiO₂ supported SMA catalysts were prepared for the partial oxidation of methane at 873 K in the presence of excess water vapor. It was found that SMA still remained on the catalysts and worked well as active species for partial oxidation of methane to formaldehyde. It was concluded that water vapor depressed the decomposition of SMA into SiO₂ and MoO₃ and enhanced the activity for partial oxidation of methane. At the fraction of water vapor higher than 50%, the degree of methane conversion increased up to 20% with the formaldehyde selectivity of 90%, corresponding to 18% yield of formaldehyde.