Ionic diffusion through thick beds of charged matrices occurs in many fields. Ionic transport is driven by electrochemical potential gradient, i.e., it is a combined result of chemical potential gradient and electrical potential gradient. To clarify a number of unexplained observations, the steady state diffusion of NaCl and LiCl solutions through 76-mm-thick Na- and Li-bentonite pastes, reported by Dull and Low, has been critically analyzed. The analyses showed that in the pastes the concentrations of M+ ions are much higher than those of Cl- ions. The phenomenological diffusivity of cation and anion are the same. Decomposition of the phenomenological ion flux into parts due to chemical and electrical potential gradients shows that the chemical diffusivity of the anion is higher than its phenomenological diffusivity. Cations are the other way round. The electric potential gradient created by the concentration gradient of ions causes these changes. Phenomenological and chemical diffusivity are concentration-dependent and are related by an equation of the type Y = A - Brootc, where A and B are constants and c is the molar concentration of the ion. This relation is due to Coulomb's law modeling of ion-ion interaction. The above inferences have bearing on the fields of ground water contamination, durability of cement-based materials, construction of clay-lined waste landfills, and construction of nuclear deposits. (C) 2003 Elsevier Inc. All rights reserved.