Decomposition of a wood block in sub- and supercritical water was studied by directly observing the change in the size and shape of the wood block in a flow-type reaction cell through sapphire windows attached to the cell. Quantitative kinetic analysis was performed for "hinoki" (a coniferous tree, Chamaecyparis obtusa) as a typical wood sample at a temperature of 523-703 K and a pressure of 10-25 MPa. The phenomenological rate of the size shrink could be analyzed on the basis of two first-order reaction terms. Together with the measurements of the emission of total organic carbons (TOC), it was concluded that the wood block shrank with emitting TOC, and then it shrank at a smaller rate with emitting much smaller amounts of TOC. In the subcritical region, the phenomenological first-order rate constant for the initial shrink increased with the reaction temperature and approximately obeyed the Arrhenius equation with an activation energy of ca. 120 kJ mol(-1). However, the rate constant decreased suddenly near the critical point and again increased with the temperature at the higher temperatures. The phenomenological kinetics was apparently determined by a certain chemical reaction. When the decomposition reaction was assumed to be proton-catalyzed, probably hydrolysis, the rapid change in the vicinity of the critical point was reasonably understood by taking into account the remarkable decrease of the ionic product.