Complete Ni 2 exchange of a single crystal of zeolite X of composition Na 92 Si 100 Al 92 O 384 per unit cell was attempted at 73°C with ¯owing aqueous 0.05 M NiCl 2 (pH 4X3 at 23°C). After partial dehydration at 23°C and %10 À3 Torr for two days, its structure, now of composition Ni 2 (NiOH) 35 (Ni 4 AlO 4 ) 2 (H 3 O) 46 Si 101 Al 91 O 384 per unit cell, was determined by X-ray diraction techniques at 23°C (space group Fd " 3, a 0 24X7885 # A). It was re®ned using all intensities; R 1 0X080 for the 236 re¯ections for which F o b 4rF o , and wR 2 0X187 using all 1138 unique re¯ections measured. At four crystallographic sites, 45 Ni 2 ions were found per unit cell. Thirty of these are at two dierent site III H positions. Twenty of those are close to the sides of 12-rings near O±Si±O sequences, where each coordinates octahedrally to two framework oxygens, to three water molecules which hydrogen bond to the zeolite framework, and to an OH À ion. The remaining 10 are near O±Al±O sequences; only three members of a likely octahedral coordination sphere could be found. In addition, two Ni 2 ions are at site I, eight are at site I H , and ®ve are at site II. Forty six H 3 O ions per unit cell, 24 at site II H and 22 at site II, each hydrogen bond triply to six rings of the zeolite framework. Each of the 22 H 3 O ions also hydrogen bonds to a H 2 O molecule that coordinates to a site III H Ni 2 ion. Six of the eight sodalite cages each contain four H 3 O ions at site II H ; the remaining two each contains a tetrahedral orthoaluminate anion at its center. Each tetrahedral face of each orthoaluminate ion is centered by a site I H Ni 2 ion to give two Ni 4 AlO 4 clusters. The ®ve site II Ni 2 ions each coordinate to a OH À ion. With 46 H 3 O ions per unit cell, the great tendency of hydrated Ni 2 to hydrolyze within zeolite X is demonstrated. With a relatively weak single-crystal diraction pattern, with dealumination of the zeolite framework, and with an apparent decrease in long-range Si/Al ordering likely due to the formation of antidomains, this crystal like others treated with hydrolyzing cations appears to have been damaged by Ni 2 exchange and partial dehydration.