The results of studies of mass transfer through a polyamide composite reverse-osmosis membrane in the demineralization of dilute solutions of six strong electrolytes are summarized. The relation of the electrolyte concentration at the surface of the reverse-osmosis membrane, at which its true selectivity tends to zero, to the specific water flux and the sum of the hydration numbers of the cations and anions of this electrolyte has been revealed. It has been shown that at a low salinity of the feed solution, the product of the molar electrolyte flux and the sum of the hydration numbers of its cations and anions for the test membrane is almost independent of the electrolyte type. An analysis of the dependence of the solute flux on the transmembrane pressure, concentration, and temperature in the electrolyte concentration range of 0.2 to 20 mol/m(3) in the feed solution, as well as the experimentally determined activation energy of the NaCl flux, has led to the conclusion that there is an additive component of the electrolyte flux, whose contribution to the total flux is insignificant at high concentrations and becomes determining at concentrations below 2-5 mol/m(3).