Based on theorical analysis of existing models and on experimental observations, a model that allows us to connect the permeate flux to the intrinsic separation factor taking account of the terms of the minimal and maximal separation factors, was established. The experiments covering a large range of permeate flux (from several 10(-9) mu L/(m(2).s) to several 10(-3) mL/(m(2).s)) permit us to determine the values of maximal separation and minimal separation, as well as the solute transport parameter for the solutions of sodium chloride and sodium sulfate.