The flow pattern around a carbon particle (similar to3.5 mm in diameter) during the supercritical water oxidation (SCWO) process (25 MPa, 723 K, 3.6 wt % O-2 concentration) was visualized for the first time by the use of Schlieren optics and a SCWO reactor equipped with transparent sapphire windows. To link Schlieren images with flow fields, computational fluid dynamics (CFD) simulations were executed, assuming temperature-dependent density of supercritical water and heat of a surface reaction, and then the obtained density fields around the particle were numerically converted to Schlieren images. Numerically obtained Schlieren images agreed well with observed images, confirming that the CFD simulation was able to predict the flow field in SCWO processes. Such a combined approach of CFD and flow visualization revealed that, in SCWO of solid substances with a fast reaction rate, the heat of surface reaction increased the temperature of the fluid and induced an upward flow from the particle with higher fluid temperature than that of the surroundings. Such flow can increase reaction rates by enhancing the mass transfer of O-2 to solids. Therefore, a flow field in SCWO is an important factor controlling the progress of reaction and it can be reasonably predicted by CFD simulations. (C) 2004 American Institute of Chemical Engineers.