Separation of multicomponent mixtures, in particular ternary systems, was investigated theoretically and experimentally. The interactions in various multicomponent mixtures were determined. Using data from literature as well as the limited data obtained in our laboratory, we identified the situations where the multicomponent effects are strong. A correlation between the coupled permeability coefficients and straight permeability coefficients and concentration of components in ternary systems was obtained. Capabilities of thermodynamic models in describing the electrolyte-nonelectrolyte systems consisting of a weak acid were also investigated. The results of this work show that the multicomponent effects can be used in improving the separation properties of membranes. The findings of this study may be applied to several processes including recovery of small amounts of metals from wastes, separation of ionic compounds from organic, and separation of weak and strong electrolytes.