In the presence of supercritical water, organic materials are converted into energetic gas, with high hydrogen content. In the present application, thermochemical conversion of humid wastes from wine or sugar production is reached at temperatures and pressures up to 550°C and 250 bars. Material is a key point for the development of SCWG process operating in such conditions. In supercritical water the durability of metals and alloys is limited by corrosion, but ceramics and ceramic composites are known to be stable under extreme conditions. For this reason, we have proceeded to an examination of a selection of ceramic materials that could be used as an anticorrosive wall of the gasification reactor. We show that, ultra-high temperature ceramics Si3N4, BN, Al2O3 and SiC are strongly corroded by humid waste, but carbon-based materials are resistant in both subcritical and supercritical conditions in the presence of pure or waste water. As a conclusion, graphite may be a very good material for SCWG reactor.