The influence of permeation flux induced by tiny pressure gradients on the counter-current gas diffusion in porous solids is discussed. Binary counter-current gas diffusion in porous samples with a broad range of pore radii (78 nm-21 mu m) was studied in arrangements of the Wicke-Kallenbach and Graham's diffusion cells. A small pressure difference between the cell compartments was observed both in the Wicke-Kallenbach cell (7 Pa) and Graham's cell (9 Pa). For samples with pore radii above 7 mu m significant deviations of diffusion fluxes were observed. Limits of pore sizes above which the additional permeation transport influences the diffusion results beyond acceptable level were obtained by simplified calculations. The distorted diffusion fluxes can be numerically freed from the contribution of permeation flux if permeation characteristics are available. (c) 2008 Elsevier Ltd. All rights reserved.