Water sorption isotherms of gel-type Dowex 50WX4 and Dowex 50WX8 and macroreticular Amberlyst-15 resins in H + , Na + , and alkaline earth metal ionic forms have been determined at 298 ( 1 K using the isopiestic technique. The hydration numbers of cations derived from the analysis of the water sorption isotherms using the D'Arcy and Watt equation compare well with those reported for similar types of ion exchangers obtained using other techniques. An interesting feature that has emerged from the analysis is the presence of a constant amount of water having bulk structure in Amberlyst-15, irrespective of the nature of the cations, possibly those filling the pores of the exchanger. The present study, thus, indicates the structural differences between the water present in the gel phase and in the pores of the macroreticular Amberlyst-15.