A film theory/ion coupling model combining both concentration polarization/back diffusion and ion coupling phenomena for high recovery diffusion-controlled membrane systems was theoretically developed and used to predict permeate water quality for multi-solute membrane systems. The concentration polarization factor (Y) was modeled as functions of operating pressure and recovery. The individual solute mass transfer coefficients (MTCs) in multi-solute systems were found to be a function of feed water composition and were determined using solute MTCs obtained from single- and bi-solute experiments.