An experimental system was constructed in order to measure the diffusion coefficient in three types of porous media, namely mica, sodic montmorillonite, and natural compact clay. Several salts at various concentrations were used for the measurements in order to investigate the influence of these factors. Influence of porosity was also studied. In a first approximation, all the results can be summarized by a simple Archie's law independent of the clay and of the solute. The diffusion and electric formation factors have also been systematically compared; they generally agree for large porosities, while they disagree for small porosities for clay and montmorillonite. (C) 2003 Elsevier Science (USA). All rights reserved.