A new method has been developed for determination and analysis of diffusion and distribution coefficients of salts in dialysis and RO membranes based on unsteady-state dialysis and steady-state RO experimental data. The method is based on a two-part model of heterogeneous RO membranes consisting of perfect or dense and imperfect or porous parts. Such a model can be described by differential equations accounting for all water and salt fluxes entering and leaving the membrane. The method was applied to various salt solution/polymer membrane systems. Thermodynamic and kinetic parameters of the system were found to be influenced by physico-chemical parameters of the penetrant reflecting its ionic properties, chemical composition of the polymer as well as structure the membranes.