This study models the effect of the electrical diffuse layer at the pore matrix interface on ionic diffusion processes in a porous medium in presence of a concentration gradient. We find that the equation D=Df/Fϕ usually used to compute in steady-state conditions the effective diffusion coefficient, D, of a porous medium as a function of the salt diffusion coefficient, Df, the electrical formation factor, F, and the porosity, ϕ, should be replaced by: D=(Df/Fϕ)&xgr;, in the presence of an electrical diffuse layer at the pore-matrix interface. The correction factor, &xgr;, arises from the cross-coupling of ionic fluxes due to electroneutrality requirement. ¿ American Geophysical Union 1996