Abstract: A mathematical model which describes the diffusion behavior and the cross-effects between ternary electrolyte mixtures in porous particles has been developed. The results of the model show that the component, with noinitial concentration gradient, still could diffuse with large flux resulting from the cross effects. When the simple Ficks law is applied to the diffusion of electrolyte mixtures, the effective diffusivities defined by the law are apparent effective diffusivities represented as weighted averages of the main effective diffusivities. The differences between the apparent and the main effective diffusivities depend on the cross effect diffusivities to a great extent. The cross effects will be remarkable if the ions in an electrolyte molecule have very different mobilities. Experiments were designed to obtain the effective diffusivities for HCl-KCl-H2O system. The model developed in this paper can be used to elucidate the internal diffusion behavior of ternary electrolyte systems.