Experimental results of a supercritical water oxidation (SCWO) system with a transpiring wall reactor (TWR) containing a turbulent hydrothermal methanol diffusion flame as internal heat source are presented. The present paper focuses on the hydrothermal flame combustion in a geometrically simple wall-cooled coaxial burner. Various kinds of hydrothermal flame experiments with methanol-water mixtures as fuel stream and pure oxygen as oxidizer were conducted at an operating pressure of 250 bar. Ignition was accomplished by heating-up the reactants to auto-ignition temperature (typical ignition temperatures 460-490 degrees C). To investigate the flame stability, the injection temperature of the methanol-water mixture was lowered step-by-step to extinction. For methanol mass fractions higher than 11%, desired subcritical extinction temperatures are reached. The inlet temperature of the fuel stream can be lowered below 100 degrees C with 27 wt.% methanol. The conversion ratios based on the analysis of the liquid effluent are generally above 99.95% for flame residence times in the order of magnitude of 100 ms. (C) 2008 Elsevier B.V. All rights reserved.