The diffusion induced by the internal driving force of the concentration cell was shown to be the mean average of the drift speed and steady-state ion diffusion. Based on this observation, a model containing both drift speed and steady-state diffusion was derived. The model contains only known parameters. The drift speed and steady-state diffusion were measured in a ceramic macro-porous membrane. By comparing with, among the drift speed, steady-state diffusion and the new model for diffusion, the new model showed its reliability. More consistent results were obtained and the contributions involved in the diffusion were much more visible. The new model was successfully tested with NaCl, NaOH and Na2S.