Details of the anion-exchange properties of Na-montmorillonite modified with hexadecylpyridinium chloride (HDPyCl-montmorillonite) for perchlorate ions are reported herein. The perchlorate uptake on HDPyCl-montmorillonite from 0.01mmol/dm 3 or 0.10mmol/dm 3 solution rapidly attained equilibrium within 4h. The perchlorate uptake on HDPyCl-montmorillonite followed the Langmuir isotherm model with capacity of 1.02mmol/g. The perchlorate uptake of 0.70–0.80mmol/g on HDPyCl-montmorillonite persisted in a wide pH range of 4–10. High selectivity of HDPyCl-montmorillonite for perchlorate ion was reflected in the high distribution coefficient values (>10 4 cm 3 /g) with selectivity sequence of HPO 4 2− <SO 4 2− <NO 3 − <ClO 4 − from multi-anion solution of 0.10, 1.0, or 2.0mmol/dm 3 . This selectivity sequence corresponded to the sequence of increasing hydration energy of anions. These data revealed that the anion uptake on HDPyCl-montmorillonite accompanied with release of chloride ion into solution through anion-exchange processes.