Formaldehyde (HCHO) reactions in supercritical water (673 K and 25-40 MPa) with and without acid and base catalysts (homogeneous: H-2 SO4 and NaOH, and heterogeneous: CeO2, MoO3, TiO2, and ZrO2) were conducted by use of batch reactors. Cannizzaro reaction (2HCHO + H2O --> CH3OH + HCOOH) and self-decomposition of HCHO (HCHO CO + H-2) were found to be primary reactions for all the cases and the contribution of each reaction depended on the condition. In the case of the homogeneous systems, Cannizzaro reaction became dominant with increasing bulk hydroxyl ion (OH-). The simple network model can well express the experimental results in the homogeneous conditions. We correlated the ratio of the yield of CH3OH to that of CO (at 15 min) against bulk OH- in the homogeneous system. For elucidating acidity and basicity of metal oxide catalysts on HCHO reaction in supercritical water, OH- concentration on the metal oxide surface was calculated by use of the above correlation and the following order was found: CeO2 > ZrO2 > MoO3 > TiO2 (rutile) > TiO2 (anatase). At the reaction condition, CeO2 and ZrO2 were base catalysts, and MoO3 and TiO2 were acid catalysts. The experimental results with the metal oxides can be expressed by the model that was developed under homogeneous systems, with the values of the OH- concentrations that were calculated from the correlation about the CH3OH/CO ratio at 15 min of reaction time. (C) 2003 Elsevier Science B.V. All rights reserved.