Modelling approaches of biomass steam gasification are investigated that take into account both chemical and physical kinetic limitations. The gas phase can be described by two independent reactions: (i) the steam reforming of CH4, which is kinetically limited under the operating conditions (1073 < T < 1273 K, p = 1 bar), (ii) the water-gas shift reaction, which would be close to equilibrium under the operating conditions (1073 < T < 1273 K, p = I bar). Concerning solid, a time scale analysis of the main phenomena has been performed. For particles of 500 mu m, the transformation can be seen as two successive steps: (i) pyrolysis of biomass, which is both chemically and heat-transfer controlled; (ii) steam gasification of residue, which is chemically controlled. (c) 2006 Elsevier Ltd. All rights reserved.