Supercritical water oxidation is an efficient technology for the ultimate destruction of organic waste materials. We previously reported that the addition of activated carbon catalyst promoted the oxidation of phenol in supercritical water and that yield of tarry materials was remarkably suppressed at 400 degreesC and 25 MPa. In this study, reaction kinetics of the carbon-catalyzed phenol oxidation in supercritical water was studied, and especially, the influence of mass-transfer limitation inside and outside of the catalyst particles was investigated. Experimental results indicated that mass-transfer limitation between bulk fluid and the catalyst surface was negligible whereas mass transfer within the pores of the activated carbon catalyst limited the overall reaction rate. This was in agreement with the result of the calculation of Mears' and Weisz-Prater's criteria. We then developed model equations considering the influence of mass transfer to investigate the intrinsic reaction rate and to describe the temporal change of reaction kinetics. In the model, three reactions were taken into account: homogeneous phenol oxidation, heterogeneous phenol oxidation on the catalyst surface, and combustion of carbon catalyst. The parameter values were determined by curve fitting with the experimental data. By this model, temporal changes of the mass-transfer effect and the reaction rate profile in the packed bed were determined.