Partial oxidation of methane by molecular oxygen over Cr-and Fe-promoted, as well as unpromoted, vanadyl pyrophosphate catalysts has been studied in the temperature range of 573-698 K and atmospheric pressure. Carbon monoxide, carbon dioxide, and water were found to be the principal reaction products over unpromoted vanadyl pyrophosphate, Over both the Fe-acid Cr-promoted catalysts high formaldehyde selectivity was observed at very low methane conversion levels with HCHO space time yields in the range of 0.5 to 2.0 g/kg-h. As the extent of the reaction was increased selectivity to formaldehyde decreased rapidly and changes in selectivities with conversion indicate a sequential conversion of methane to formaldehyde, CO, then CO2, Macrokinetic parameters for the methane oxidation reaction were determined for the unpromoted catalyst. A simple power law rate expression fit the rate data well over the whole temperature range. The rate of reaction of methane was 0.08 +/- 0.02 order in oxygen partial pressure, 0.73 +/- 0.07 order in methane partial pressure, and the activation energy was found to be 102 +/- 6 kJ/mole. Activation energy was unchanged by promotion with Fe and Cr. Analysis by XPS and ICP-kcl indicates that promoters were incorporated equally into the bulk and surface of these catalysts, XPS indicates an increase in the average surface oxidation state of vanadium in the promoted catalysts and XRD shows that promotion causes oxidation of a small fraction of the pyrophosphate to form alpha(n)-VOPO4. P-31 NMR spin-echo mapping confirms the enhanced formation of V5+ in the promoted samples. The presence of V5+ may therefore be required for the formation of selective products. It is proposed that the role of promoters is to increase the rate of formation of V5+ during activation, or to stabilize V5+ containing domains under the highly reducing methane oxidation conditions.