In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH4 14%; CO 19%; CO2 14%; H-2 16%; H2O 30%; N-2 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800-1000 degrees C) and space time (1.6-6.2 min g L-1). Determination of Thiele modulus evidences a change of kinetic regime from chemically-to diffusion-controlled when the temperature increases; this finding is particularly relevant when porous chars having an average pore width of 1 nor are used as catalysts. Mass diffusion transfers are accounted for by a model introducing an internal effectiveness factor. Knudsen diffusion in micropores is shown to limit the conversion rate of methane per unit mass of catalyst, and explains why such a rate is not proportional to the BET surface area, especially when the latter is higher than typically 300 m(2)/g. It is concluded that diffusion limitations in micropores should be taken into account, otherwise underestimated activation energy and intrinsic kinetic constant are obtained in some experimental conditions. (c) 2009 Elsevier B.V. All rights reserved