[en] The adsorption of protium and deuterium oxides at 298.15 K on the lithium, sodium, potassium, and barium forms of Dowex 50W-X8 cation exchanger was studied. In the whole humidity range the resinates adsorb larger mol amounts of H₂O than of D₂O. In the low humidity range the observations fit well the two parameter B.E.T. isotherm. The Gibbs energy changes of transfer of the resinates from H₂O to D₂O were ΔGsub(t)=2400 J/mol. About 22 J/mol of this is believed to be caused by the anions. Thus, in their interactions with the isotopically different waters the resinates resemble ordinary salts. The relative magnitudes of the ΔGsub(t) values reveal different surface effects between the resin matrix bound sulfonate groups ant the two kinds of water. (author)