In this work, the advantages provided by supercritical water (SCW) to one of the most extended methods to produce H-2, the gasification of carbonaceous materials, are investigated. The broad range of experimental conditions assayed (1-1000 bar) allows not only the comparison of the use of steam and SCW but also the investigation of how H-2 production varies within the supercritical region. The results obtained reveal important advantages when the supercritical fluid is used. In the gasification with SCW, H-2 production is greater, and in general, the consumption of the raw material is optimized since more H-2 is obtained when the same amount of solid is consumed. Furthermore, the gaseous streams are very rich in H-2 and CO2 and poor in CO and CH4. On the other hand, the use of two carbonized materials of different natures (a phenolic textile fiber and holm oak wood) allows us to highlight that H-2 production depends strongly on the surface irregularities and surface chemistry of the solid. The material of vegetal origin is more suitable because more H-2 is produced and mixtures of gases that are more easily purified are obtained from it. However, higher amounts of CO2 than those registered for the solid of synthetic origin are released during its gasification.