An equation was mathematically derived using the theory of irreversible thermodynamics for the diffusion of electrolytes in solution in the presence of an external gravitational force. It was demonstrated that diffusion coefficients of electrolyte species are dependent on the difference of radii and densities of anion and cation ionized from the electrolyte in solution. For a special case that radii and densities of the anion and cation are the same, the diffusion coefficient is independent of the external gravitational force. Entropy production was also estimated for the system under consideration. It was found that the rate of entropy production is proportional to the sum of the gravitational forces induced to all electrolyte species. (C) 2003 Elsevier B.V. All rights reserved.