Selected thermodynamic and physical characteristics of ions were compared in relation to their permeation ability during membrane filtration. The characteristics analyzed included ionic radius, hydrated radius, hydration free energy, Jones-Dole viscosity coefficient (B). Correlations between ionic parameters were analyzed to explain the ability of some ions to permeate by rearranging the water molecules (i.e., soft ions) within their hydration shells during transport as well as near membrane inter-ionic interactions. Hydrated size and strength of the hydration shell significantly affect ability of ions to permeate through the thin film membranes. Selectivity of membrane processes depend on the hydration free energy of ions. Ion selectivity of a membrane (or partitioning of ions on the membrane) is governed by differences in relative free energies between the solution and membrane phases. The Jones-Dole coefficient (B) for both cations and anions show strong correlations with selected thermodynamic characteristics including entropy of hydration and hydration free energy. The order of ions with respect to selected parameters show similarities with the Hofmeister series which is often used to explain the ability of ions to interact with hydrogen in proteins. The same ability (i.e., interaction with hydrogen) affects the behavior of ions both near and within membrane, allowing some ions to permeate either by size exclusion or ability to rearrange the hydration waters under shear forces during transport. (C) 2011 Elsevier B.V. All rights reserved.