The rates of adsorption of hydrochloric acid by 16 weakly basic ion-exchange resins have been measured, together with the rates of adsorption of sodium hydroxide by 16 weakly acidic ion-exchange resins. The results obtained for the weakly basic resin "Amberlite IRA-93" confirm that the adsorption rates are inversely proportional to the square of the bead radius, and are independent of the salt concentration, as demanded by the diffusion model of Helfferich for the concentration range considered. The adsorption rates are related to the physical and chemical structures of the resins, with the porosity of the l&n beads being the most important factor in resins which are suitable for the operation of a thermally regenerable process. Other parameters studied include the e;fect of crosslinking, and the introduction of hydrophilic groups into the resin network. It was found that, even with the most porous resins available. the rates obtained are inadequate ior the practical o_peration of the prcms if resin beads of normal size are to be used. New methods of handling microbeads. or novel mixed resin systems of normal particle size but involving short diffusion paths, are therefore necessary.