The crystal structure of an iodine sorption complex of fully dehydrated, fully Cd2+-exchanged zeolite X, Cd46Si100Al92O384 . 89.6I per unit cell (a = 24.861(8) Angstrom), has been determined by single-crystal X-ray diffraction methods in the cubic space group Fd (3) over barm at 21(l) degreesC. The crystal was prepared by ion exchange in a slightly basic aqueous stream 0.05 M in Cd2+, followed by washing with distilled water at 80 degreesC. Dehydration was then done at 450 degreesC and 2 x 10(-6) Torr followed by exposure in situ to zeolitically dry iodine vapor. The structure was refined to the final error indices R-1 = 0.044 and R-2 = 0.043 with 263 reflections, for which I > 3sigma(I). Forty-six Cd2+ ions are found at four crystallographic sites: 14 at site I at the center of the hexagonal prism (Cd-O = 2.389(5) Angstrom), 4 at site I' in the sodalite cavity just outside the hexagonal prism (Cd-O = 2.441(8) Angstrom), 2.4 at site II' in the sodalite cavity (Cd-O = 2.200(9) Angstrom), and the remaining 25.6 at site II near single six-oxygen rings in the supercage (Cd-O = 2.218(7) Angstrom). All 44.8 diiodine molecules sorbed per unit cell have disproportionated to give 12.8 n-I-5(-) anions with symmetry 2 and 6.4 square cyclo-I-4(2+) cations. The sorption reaction ends precisely when the eight supercages per unit cell are full (12.8/2 + 6.4/4 = 8.0). In cyclo-I-4(2+), I-I = 2.757(14) Angstrom and 2.796(13) Angstrom, and I-I-I = 90.0(5)degrees; each iodine atom is 3.167(10) Angstrom from a framework oxygen (I-I...O = 159.4(4)degrees). In n-I-5(-), the central bond lengths are 2.473(14) Angstrom and the terminal bond lengths are 2.534(13) A, all less than 2.67 Angstrom, the I-I distance in I-2. The central bond angle, 114.8(7)degrees, is indicative of I-3(+), indicating that the terminal atoms are iodide ions. That the terminal atoms coordinate to Cd2+ also indicates that they are iodide ions. The 12.8 n-I-5(-) anions per unit cell each bridge between two site-II Cd2+ ions in the supercage.