A methodology for the analysis of local pore size in an arbitrary porous medium, based on the maximum sphere inscription method, has been developed. The method has been validated for regular cylindrical pores and used for the evaluation of pore size distribution on several random media, both computer-generated and scanned using X-ray microtomography. The local pore radii have then been used for the calculation of effective diffusion across the porous medium, whereby local Knudsen diffusion coefficients were employed depending on the local pore radius. This continuum-based approach was then compared with the results of Knudsen diffusion through the same porous media simulated by the molecular dynamics method. A difference of maximum 40% was obtained, which is deemed acceptable given the significantly faster computing speed and higher versatility of the continuum-based models. (C) 2009 Elsevier Ltd. All rights reserved.