Results and analysis of the oxidation of methane in supercritical water and argon by oxygen over a pressure range from 13 to 28 MPa and temperature range of 390°C to 440°C are presented. Raman spectroscopy is used as an in situ diagnostic to monitor the concentration of methane, oxygen, carbon monoxide, and carbon dioxide in a constant volume reactor. The results from these experiments have identified several unexpected characteristics in this temperature range. Specifically, we have observed an unexpected decrease in reaction rate with increasing pressure. In addition, we have found that the existing elementary models do not predict accurately the concentration-time profile for CO and underpredict the formation of formaldehyde. Although more experimental work and analysis needs to be done, we suggest that this pressure dependence originates from the competition between two key elementary reactions that directly involve the water molecule.