In order to improve the understanding of biomass gasification in a bed fluidized by steam, the thermochemical equilibrium of the reactive system was studied. The equilibrium results were compared to LGC experimental results, obtained by the gasification of oak and fir in a laboratory-scale fluidized bed of different catalysts: sand, alumina, and alumina impregnated with nickel. The research was completed by a study of the influence on the equilibrium of additional parameters such as the quantity of steam, the pressure or the kind of biomass. Those results of simulation may be used for evaluating the limits of actual reactors. The following conclusion may be drawn from all the results: The thermodynamic equilibrium state calculated is far away from the experimental results obtained on sand particles. The steam to biomass ratio, between 0.4 and 1 kg(steam)/kg(dry) biomass. has a strong influence on the gas mixture composition. The temperature increase and the use of catalyst allow producing a gas mixture with a high content of hydrogen and carbon monoxide. The H-2:CO ratio may reach values greater than 3. The use of catalyst allows the system to get closer from the equilibrium, especially for the nickel based catalyst. (C) 2010 Elsevier B.V. All rights reserved.