Noncatalytic reaction pathways and their rates of acetaldehyde were determined in the neat system and in supercritical water at 400 degreesC and the density of 0.1-0.6 g/cm(3). In supercritical water, acetaldehyde undergoes five types of reactions: (i) decarbonylation into methane and carbon monoxide, (ii) self-disproportionation producing ethanol and acetic acid, (iii) cross-disproportionation generating ethanol and carbonic acid, (iv) condensation forming crotonaldehyde, and (v) the subsequent polymerization of crotonaldehyde, or its decarbonylated monomer. Reactions i and iv proceed irrespective of the presence of water, while water suppresses reaction i. Reactions ii and iii are characteristic of aldehyde under hydrothermal conditions. Although reaction ii produces the same products as the classical Cannizzaro reaction, it does not require any added catalysts. Reaction iii manifests the role of formic acid as a reducing aldehyde. Actually, it is shown that reaction iii involves a larger weight than reaction ii and leads to the excess production of ethanol. The rates of these reactions are sensitive to the water density, and path weight control is thus possible through variation of the thermodynamic conditions. New reaction mechanisms are proposed for the present set of high-temperature processes.