AbstractThis article reviews the work relating to the supercritical water gasifi cation of biomass with a focus on hydrogen production. The high hydrogen yield predicted by thermodynamic calculations and the special properties of near‐ and supercritical water support the biomass degradation; these were the main reasons why the process of = supercritical water gasifi cation was investigated. The main advantage is that biomass, with a natural water content of 80 wt.% or more, can be converted without drying before. The energy required for heating up the relatively high water amount can be recovered by a compact heat exchanger, which is very important for the overall energy balance. The chemistry of biomass degradation is rather complex: from experiments with model compounds, the main reaction pathways and their dependencies on reaction conditions are identifi ed. This knowledge was applied in studies of biomass conversion. Biomass may include proteins and salts, which have a signifi cant infl uence on the gasifi cation: salts increase and proteins decrease the gas yield at comparable reactions conditions. In addition, the heating‐up rate and the reactor type used infl uence the results. For the scale‐up in view of a technical application, a bench‐scale plant is necessary. This plant exists for some years and demonstrates the process feasibility also in the scale of 100 kg/h. Still challenges for a technical application, like corrosion and solid handling, exist. © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd