The behaviour of an integrated-planar solid oxide fuel cell (IP-SOFC) strongly depends on the reactive diffusive flows within its porous support structure. Fuel is transported through the porous structure to the anodes of the electrochemical cells and the structure may be impregnated with the required catalysts for the steam reforming of methane. It is important to be able to calculate the distribution of gas properties within the porous structure in order to predict the performance of each cell and to determine the amount of internal reforming that takes place. This paper describes a three-dimensional numerical calculation method which has been developed to solve the governing equations in the porous structure. The calculation method includes the interaction between the flow in the porous medium and that in the adjacent fuel supply channel. The results highlight the importance of the kinetics of the reforming reaction and the thermal boundary conditions, both of which have a significant effect on the flow field within the porous structure. (C) 2004 Elsevier Ltd. All rights reserved.