Supercritical water gasification (SCWG) of biomass was studied by means of a stoichiometric thermodynamic model. Such approach is based on reaction equilibria and it allows calculating the extent of the single reactions involved. A very simple model was proposed, involving only 6 components and 3 independent reactions. The model was validated against literature experimental data for different feedstock and it was used to understand the influence of biomass and operating conditions. The analyses allowed calculating the values of the reaction extent of water-gas shift (WGS) and CO methanation at equilibrium: a graphical approach was proposed in order to interpret the outcomes. Results showed that the influence of pressure is very limited. Moreover, it was observed that the maximum H-2 production is linked to a parameter psi, function of the H/C and O/C ratios of the feedstock. Finally, considerations about the role of WGS and methanation in the production of H-2 were traced. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.