The flow behavior within vessels containing supercritical water has traditionally been very difficult to measure and to model, due to the highly variable fluid properties and the thick walls needed to contain the high pressures involved with supercritical water. Using neutron radiography, we are able to perform a water-in-water ((H2O)-H-1-in-D2O) tracer measurement of a supercritical-water reverse-flow vessel that has been designed for salt precipitation in a hydrothermal biomass gasification process. With this method, we have discovered that flow patterns within the vessel are quite disperse and are strongly accelerated by buoyant forces. This method has the beneficial feature that measurements can be taken that are completely non-invasive (for suitable vessel materials, such as zirconium alloys) and do not require the presence of windows or thermowells. We expect that this method will be applicable to a range of research, from reactive processes to nuclear power heat transfer applications, especially when multiple phases, density gradients, or reactions that cause precipitation are present. (C) 2010 Elsevier B.V. All rights reserved.