The specific surface reaction rates of Breasol coal and styrene-divinylbenzene resin particles in sub and supercritical water with O-2 were investigated in the temperature range of 300-450 degrees C at 25MPa by observing the size change using a shadowgraph method. The rates were approximately proportional to the O-2 concentration and non-linearly dependent on the flow rate. The rate obeyed the Arrhenius law at lower temperatures, but became gradually independent on the temperature above 400 degrees C, which indicated that the overall reaction is controlled by a certain mass transfer process at the higher temperatures. Compared with the case of coal and activated carbon, much smaller amount of O-2 is enough to result in the rapid size decrease rate in the resin particle. (c) 2007 Elsevier Ltd. All rights reserved.