A single relaxation of order 0.1 s was observed in a carboxymethyl (CM) Sephadex C-25 suspension containing ammonia by the pressure-jump method with electric conductivity detection. The relaxation time became fast with increasing concentration of added ammonia. Ammonia was strongly adsorbed on the CM Sephadex particle, which had a soft spongy structure consisting of flexible dextran chains. The relaxation was attributed to the conformational change of the Sephadex particle accompanying the adsorption-desportion of ammonium ions.