Rates of salt diffusion through homoionic clay plugs were determined after the plugs had been brought to steady state between solutions of the chloride of the cation with which the clay was saturated. Salt and water contents were measured across the clay plugs and were used with the diffusion rates to calculate porous system diffusion coefficients.Diffusion coefficients increased as an exponential function of water content, but for practical purposes, were independent of the salt concentration.Diffusion potentials were measured and calculated. They indicate that the adsorbed cations contribute slightly to the movement of the salt cations.