Although neutron radiography has been effectively used to dynamically image the separation of salts from supercritical water, earlier approaches have relied on an artificial model system that was limited to boron-containing salts in heavy water, D2O. In this work we demonstrate that similar information may be obtained with the contrasts reversed: boron-free salt buildup can be seen in supercritical H2O. This new technique enables in situ imaging as illustrated by results obtained for systems of interest for biomass gasification, where solid plugging and flow pattern changes were clearly imaged within the vessel. As compared to the D2O method, the H2O method does not explicitly distinguish between regions of low water density and buildup of salt; however, the H2O method can provide information on fluid density changes that the D2O method does not. This method was applied to determine the behavior of Na2SO4 in (H2O)-H-1, which led to plugging and erratic behavior, and to K3PO4 in (H2O)-H-1, which led to smooth operation. Plugging behavior of Na2SO4 was drastically reduced by the addition of K3PO4 to the feed, which suggests the formation of a liquid brine. (C) 2008 Elsevier B.V. All rights reserved.