The pressure-jump technique with the electric conductivity detection was applied following the adsorption-desorption of (\mathrm{Li}^{+}, \mathrm{Na}^{+}), and (\mathrm{K}^{+})ions in the suspension of a cation exchanger, carboxymethyl (CM) Sephadex C-25, and a single relaxation of (0.1 \mathrm{~s}) order was observed. The relaxation times became fast with increasing concentrations of the added (\mathrm{LiOH}, \mathrm{NaOH}), and (\mathrm{KOH}). Under the same conditions as those in the relaxation measurements, alkali metal ions were strongly adsorbed on the Sephadex C-25. The selectivity of alkali metal ions on the Sephadex surface increased in the sequence (\mathrm{Li}^{+}<\mathrm{K}^{+}<\mathrm{Na}^{+}). The (\zeta) potentials of the Sephadex particles were negative in the presence of the bases, and the negative values increased in the sequence as (\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Li}^{+}). This indicates that the stronger adsorption counteracts the more negative charge on the particle surface. On the basis of these data, the relaxation was attributed to the conformational change of the Sephadex particles accompanying the ion-pair formation between alkali metal ions and the dissociated CM groups. Since the rate constants of the conformational change increased with a decrease in the atomic weights of alkali metal ions, the conformational change was interpreted as movement, such as the vibration of the dextran chains of the Sephadex C-25. (1994 Academic Press, Inc.