A physic-mathematical model is presented for multiphase diffusion mass-transfer in shell-core type ion exchanger. Both liquid film diffusion and inner-particle diffusion

which control the mass-transfer rate

are unsteady-state processes in quasihomogeneous phase. An equation of exchange fraction is. systematically derived. The basic laws of kinetics offer a theoretical basis for selecting optimization parameters. This equation can also be used to determine inner-particle diffusion coefficient and liquid film thickness.