A novel supercritical water oxidation (SCWO) system with a transpiring-wall reactor (TWR) containing a hydrothermal flame as internal heat source has been investigated. This configuration may overcome the problems of corrosion and plugging of reaction vessels and other components. In the TWR, the wall contact of supercritical reaction media is prevented by fluid dynamical means. Subcritical water permanently flows through porous, non-load-bearing transpiring-wall elements and forms a "film" on the inner wall. Precipitated salt particles do not reach the wall because they are either re-dissolved in this film or are swept away by the transpiring water. The waste water reaches reaction temperature without any wall contact, i.e., by mixing with the combustion products of the hydrothermal flame. In the present work, the apparatus design, processing features, and experimental results are presented. Tests with salt-free water-methanol mixtures have been conducted at 250 bar. The influence of the intensity (i.e., mass flux) and temperature of transpiration on the reactor performance has been examined. The temperatures near the transpiring wall remain subcritical which is a quantitative measure for the wall protection from salt adherence. Methanol conversions of higher than 99% were obtained even at comparatively low transpiring water temperatures (125-250 degrees C). At typical operating conditions, the mass flow ratio between the total transpiration flow and the core flow is around 65%. Further, it was found that natural convection effects are not negligible in a transpiring-wall reactor for SCWO. (c) 2004 Elsevier B.V. All rights reserved.