The partial oxidation of methane to formaldehyde was studied on K2MoO4, deposited on various supports, in a fixed-bed continuous-flow reactor at 860-923 K using O-2 as oxidant. The catalysts were characterized by Raman and XPS spectroscopies. It was found that the composition of molybdates deposited by impregnation depended sensitively on the pH value of the slurry containing the support. At low pH values a significant amount of K2Mo2O7 was formed. The product distribution of the oxidation reaction was markedly influenced by the nature of the support. The highest activity was measured for magnesia-supported catalyst. In this case, however, only the complete oxidation of methane occurred. Formaldehyde, in a larger quantity, was produced on silica-supported catalyst containing a greater amount of K2Mo2O7 and on K2MoO4/ZSM-5. It was observed that at very low oxygen content, similar to 0.1%, the reaction pathway of methane conversion was basically different : in this case the main hydrocarbon products were ethylene, ethane, and benzene. Formaldehyde was not detected. A possible mechanism for the reaction of methane is discussed.