In supercritical water at 400degreesC and 0.5 g/cm(3) (37 MPa), acetaldehyde was found to be transformed without catalysts into ethanol and acetic acid through a Cannizzaro-type disproportionation reaction. No aldol-type condensation products were detected, and the disproportionation competed with the decomposition into methane. Ethanol was generated in excess to acetic acid, and carbon dioxide appeared as a product only in the presence of supercritical water. This reaction behavior is explained by considering that carbon monoxide is provided by the acetaldehyde decomposition and leads to the formation of formic acid as a reducing intermediate for acetaldehyde.