The effect of additives, such as an inorganic alkali and a nickel catalyst, on the hydrothermal process was examined to generate hydrogen from biomass with high selectivity at relatively low temperatures around 400 degrees C. At first, a cellulose sample as model biomass was subjected to the hydrothermal process at 400 degrees C under 25 MPa in the presence of an alkali (Na2CO3) and a nickel catalyst (Ni/SiO2). The combination of these two additives led not only to highly efficient generation of hydrogen but also to effective dissolution of CO2 into an alkaline liquid layer. Here the molar yields of gas products from the cellulose sample were compared with the equilibrium quantities obtained using a thermodynamics calculation software. Furthermore, the hydrothermal process of real biomass, such as wood chips, organic fertilizer and food waste, in the presence of both the two additives resulted in highly selective production of hydrogen even at 400 degrees C. (C) 2008 Elsevier Ltd. All rights reserved.