The partial oxidation of methane to formaldehyde by molecular oxygen on silica and silicasupported oxide catalysts has been investigated at a pressure of (1.7 \mathrm{bar}) in the temperature range (520-650^{\circ} \mathrm{C}) by using a batch reactor with external recycle. The effects of reactor diameter, recycle flow rate, catalyst weight, and methane-to-oxygen ratio on the catalyst activity have been outlined. By performing several blank tests with an empty and a quartz-filled reactor, it has been demonstrated that the gas-phase reaction does not affect the catalytic pathways. Reasons for controversial results reported previously are discussed. They lie in the lack of an adequate experimental approach and in the generally adopted rule to evaluate the catalytic activity at differential conditions in order to push the HCHO selectivity to high values. The approach presented here allows to evaluate the catalytic activity by performing tests at quasi-zero conversion per pass but at a finite extent of conversion. The need to express the catalytic activity as space time yield (STY) to HCHO (\left(\mathrm{g} \cdot \mathrm{kg}{\text {cat }}^{-1} \cdot \mathrm{h}^{-1}\right)) is presented. The reactivities of various commercial (\mathrm{SiO}{2}) samples obtained by precipitation, sol-gel, and pyrolysis methods have been determined. The fact that the nature and source of silica has a marked effect on (S T Y), previously observed for reaction at (520^{\circ} \mathrm{C}), has been confirmed for operation at (550-650^{\circ} \mathrm{C}). Highest (S T Y \mathrm{~s}) are found with precipitated silica samples. In fact, at (650^{\circ} \mathrm{C}) with such precipitated (\mathrm{SiO}{2}) a (S T Y) to (\mathrm{HCHO}) of (303 \mathrm{~g} \cdot \mathrm{kg}{\mathrm{cal}}^{-1} \cdot \mathrm{h}^{-1}) has been obtained. Incorporation of molybdena depresses the (S T Y) value for the precipitated silica but enhances the (S T Y) of bare fumed silica. In contrast, addition of vanadia to either precipitated or fumed silicas leads to higher STY values, 1993 Academic Press. Inc.