Abstract The adsorption of water vapour on anion-exchangers of various degrees of cross-linking ( × 2, × 4, × 8, × 10) and in different ionic forms (Cl −, Br −, I −) was studied by the isopiestic technique. The calculated integral free-energy changes were independent of the degree of cross-linking of the resins. With increase in the number of adsorbed water molecules the free-energy functions approached limiting values which were characteristic for the counter-ions. The free-energy change was combined with the enthalpy of water sorption (obtained from direct calorimetric measurements) to obtain the entropy change due to the water uptake. Both the enthalpy and the entropy functions indicated the existence of several processes during the adsorption of water, among which the most relevant are hydration, swelling of the matrix, and dilution of the internal electrolyte of the ion-exchanger.