Ni2+-exchanged zeolite Y was prepared by solid-state ion-exchange methods (Ni30Na7Cl12Si137Al55O384. nH(2)O per unit cell) and was fully dehydrated (Ni-30-Y). Rehydration with D2O and evacuation at 24 degrees C yielded a second sample (Ni-30-Y . D2O). Pulsed-neutron diffraction data were collected at 15 K : the structures were determined in the space group Fd3m; R-p = 0.0212 and 0.0184 and R-up = 0.0287 and 0.0257 for the two structures, respectively. In Ni-30-Y, 30 Ni2+ ions occupy 4 crystallographic sites : four ions are found at site I, at the centers of double 6-rings; 18 occupy two I＇ sites. outside double 6-rings. 2.09(1) and 2.50(2) Angstrom, respectively, from three nearest-framework oxygens : 8 are located at site II＇, adjacent to a single 6-ring, 2.13(1) Angstrom from three nearest-framework oxygens. Sites I＇ and II＇ are both 3-fold-axis sodalite-unit positions. The 12 Cl- ions each bridge between two site-I＇ Ni2+ ions (Ni-Cl = 2.60(1) and 2.73(4) Angstrom); each Cl- ion is 2.93(2) Angstrom from two framework oxygens and 3.14(2) Angstrom from two other framework oxygens. Six of the eight sodalite units per unit cell contain a (NiClNiClNi)(4+) cluster. The two remaining sodalite units each contain four Ni2+ ions tetrahedrally arranged at site II＇. In Ni-30-Y . D2O, the 30 Ni2+ ions are located as follows : 4 at site I, 11 at site I＇. 4 at site II＇, and 11 at site III＇ in the supercage. Five of the sodalite units contain one Ni(I＇) ion, each coordinated octahedrally to three D2O molecules and to three framework oxygens; three sodalite units contain two octahedral Ni(I＇) ions, each coordinated to one terminal D2O molecule, two bridging D2O molecules, and three framework oxygens. Each site-III＇ Ni2+ ion coordinates to four framework oxygens in a distorted square-planar manner, ia deuterium atom of a water molecule coordinated to each site-III＇ Ni2+ ion hydrogen bonds to a framework oxygen (D-O = 1.75(4) Angstrom). It is clear that the exchange of NF4+ for Na+ in the initial step of sample preparation was incomplete and that NiCl2 was imbibed by zeolite Y during the exchange process.