The structures of fully dehydrated, fully Cd2+-exchanged zeolite X, Cd46Si100Al92O384 (Cd-IB-X; a = 24.935(8) Angstrom), and that of fully dehydrated Cd2+- and Tl+-exchanged zeolite X, Cd24.5Tl43Si100Al92O384 (Cd24.5Tl43-X; a = 2 4.858(9) Angstrom), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd (3) over bar at 21(1) degrees C. Cd-46-X was prepared by ion exchange in a flowing stream of 0.05 M aqueous Cd(NO3)(2) for 2 days. Cd24.5Tl43-X was prepared similarly using a solution 0.025 M each in Cd(NO3)(2) and TlNO3. Each crystal was then dehydrated at 450 degrees C and 2 x 10(-6) Torr for 2 days, Their structures were refined to the final error indices R(1) = 0.055 and R(2) = 0.077 with 544 reflections for Cd-46-X, and R(1) = 0.054 and R(2) = 0.051 with 272 reflections for Cd24.5Tl43-X; I > 3 sigma(1). In the structure of dehydrated Cd-46-X, Cd2+ ions are located at two different crystallographic sites. Sixteen Cd2+ ions fill site I, at the centers of the double six-rings; each Cd2+ ion is octahedrally coordinated by framework oxygens, all at 2.35(1) Angstrom. The remaining 30 Cd2+ ions nearly fill the 32-fold site II in the single six-rings; each is three-coordinate planar to framework oxygens at 2.16(1) Angstrom. The fractional occupancies in dehydrated Cd Tl-24.5(43)-X are most easily explained with two types of unit cell : half have 14 Cd2+ ions at site I and four Tl+ ions at site I’. The remaining half have 15 Cd2+ ions at site I and Tl+ ions at site I’. The remaining ten Cd2+ ions occupy site II; 22 Tl+ ions extend 1.52 Angstrom into the supercage from their three oxygen planes to complete the filling of site II. The remaining 18 Tl+ ions are statistically distributed over site III, a 48-fold equipoint in the supercages on twofold axes; Tl-O = 2.79(2) Angstrom. It appears that Cd2+ ions prefer sites I and II in that order, and that Tl+ ions occupy the remaining sites, except that they are too large to be stable at site I.