Two models of a porous body, a 3D replica obtained by stochastic reconstruction and a random pore network, were evaluated with reference to the effective diffusivities of gases. Stochastic reconstruction was applied to statistical information extracted from images of 2D cuts through the porous medium. The pore network was derived in such a way that the total porosity, pore-size function, mean coordination number and mercury intrusion curve were all in good agreement with the experimental data. The simulator used to model steady diffusion flow in the pore network slightly overestimated effective diffusivity, while direct calculation of effective diffusivity, by using random-walk simulation in the 3D replica, delivered a value that was lower than its experimental counterpart. However, both deviations were within tolerable limits. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.