Partial oxidation of methane was conducted in supercritical water at O-2/CH4 ratios of 0.03 at 673 K with a flow-type reactor. The main products were CO, methanol, formaldehyde and a small amount of CO2 and H-2. With increasing water density, methane conversion increased due to the formation of formaldehyde. A free radical reaction model was applied to this reaction and it was confirmed that the dependence of selectivities for CO and CO2 on pressure could be well described. The experimental results and model prediction shows that an increase of water density shifted product distribution to partial oxidation products and formaldehyde was found to play an important role in the product selectivity and the partial oxidation chemistry. (C) 2003 Elsevier B.V. All rights reserved.