The sorption of mixed water + methanol vapours by ion-exchange resins in the sodium form has been studied gravimetrically (McBain quartz spring balance) at 25 °C, using the isopiestic method (equilibrating mixture: water + methanol + glycerol) for controlling the total vapour pressure and the vapour composition. The two resins selected for study, the gel-type Amberlite IR-120 and the macroporous Amberlite 200, give normal water sorption isotherms, but the gel resin shows irreproducible methanol vapour sorption. Systematic mixed sorption measurements were made on the macroporous resin, which presents a slight hysteresis loop for pure methanol vapour so that only rising experiments were performed. The results are analysed at constant total vapour pressure and at constant vapour composition. The mixed isotherms are, like the pure ones, of type II (B.E.T.) At saturation, capillary sorption between beds distorts the experimental results and a smooth extrapolation procedure is applied, which allows reliable data to be obtained for these states. In equilibrium, through the vapour phase to the liquid phase the resin shows a change of selectivity, preferring the minor component of the mixture. On the other hand, methanol is selected at constant vapour composition and decreasing total vapour pressure when using methanol-rich vapours.