The changes in ion-exchange characteristics of synthetic mordenite ground by a planetary ball mill under dry conditions were investigated using radioisotopes of Cs-137 and Sr-85 as tracers. The self-diffusion of Cs+ in the mordenite sample was facilitated in the initial stage of grinding, while it was leveled off for the further operation. The relative amount of Cs+ immediately exchanged in the mordenite sample was gradually styptic to a constant value with increase in grinding time after showing a rapid reduction in amount in the early stages of grinding. High selectivity of the mordenite sample for Cs+ against Sr2+ observed in acidic solution diminished obviously with an increase in grinding time.