X-ray radiography was successfully employed for following supercritical water oxidation of an isolated activated carbon particle and a packed bed of carbon particles. It was found that the size of the particle decreased with little density decrease for the activated carbon particle, contrary to the case of a graphite particle where the density decreased with almost no obvious change of the size, as previously reported. The reaction rate of the activated carbon particle was in good agreement with the previous results by means of shadowgraph observation. In the experiments for a packed bed, fluidization of the packed carbon particles was observed for the first time under supercritical conditions. The minimum fluidization velocity was found to be in agreement with conventional prediction methods. The density of packed bed was found to decrease without appreciable bed height change during supercritical water oxidation, and the rate was explainable in terms of the inner surface reaction rate of carbon particles which had been obtained previously. (c) 2005 Elsevier B.V. All rights reserved.