The objectives of this research were to determine the importance of parameters such as ionic size, ionic hydrated size, diffusivity in solution, enthalpy of hydration DeltaH and relative Gibbs (free) energy DeltaDeltaG on the salt permeation coefficient DK/delta in a RO TFC membrane. The experimental apparatus consisted of a commercial RO water maker and an in-line sensors system, which allows data acquisition on a continuous base for temperature, pH, conductivity, flow rates, and operating pressure. The solutions investigated were monovalent and divalent chlorides. Each feed solution was tested at 0.1 M. A preferential-sorption capillary-flow model PSCF was used in data evaluation. It was found that salt diffusivity, hydrated radii, enthalpy and entropy of hydration exert the controlling effect on the membrane selective transport, and any of them can be used in a comparative estimation of DK/delta. The relative Gibbs energy emerged as a parameter which can fully characterize the membrane selectivity and permits the prediction of DK/delta for a certain salt in a certain membrane.