Water uptake (n(w)) by the Mg2+, Ca2+, Sr2+, and Ba2+ ionic forms of Nafion-117 exchanger membrane as well as Dowex 50WX8 resin has been investigated as a function of water activity (a(w)). The isotherms constructed reveal appreciable differences for each ionic form, larger water uptake being seen for Nafion-117 particularly at low water activities; Thus much larger integral free energy changes on water sorption (Delta G(sw)) have been found for Nafion-117 compared to that for Dowex 50WX8, Mg2+ form revealing a factor of two while Ba2+ form Delta G(sw) involves a factor closer to three. Analysis further shows that the differences between the two exchangers emanate due to different hydration behaviour of each cation caused by different exchanger morphologies and ionic concentrations. Preliminary data obtained on the alkaline earth ion/H+ exchanges appear to support above conclusions.