The locations of extra-framework Cu2+ cations in hydrated copper ion-exchanged mordenite (formula Cu1.384T48O2 . 28H(2)O, space group Cmcm, a=18.1659(4) Angstrom, b=20.375(1) Angstrom, c=7.495(1) Angstrom, V=2774.4(4) Angstrom(3), Z=1, R=5.13%) and its partially dehydrated analog (formula Cu1.468T48O2 . 4.04H(2)O, space group Cmcm, a=18.0546(7) Angstrom, b=20.298(2) Angstrom, c=7.4772(3) Angstrom, V=2740.2(3) Angstrom(3), Z=1, R=3.90%) have been determined from the analysis of room-temperature single-crystal X-ray data. Two copper sites were found in the hydrated sample : one in the elliptical 8-ring and the other in the center of the main 12-ring channel. Upon dehydration three copper sites were determined; one in the elliptical 8-ring, one above the 6-ring in the main channel and the last in the circular 8-ring that leads into the main channel. The low coordination, but high accessibility, of the latter two cation sites in the partially dehydrated sample provides a possible explanation for the high activity of mordenite and its related pentasil family members for nitrogen oxide composition.