The crystal structures of vacuum-dehydrated, fully Mg2+-, Ca2+-, and Ba2+-exchanged zeolite X (Mg-46(H2O)(4)-, Ca-46-X, and Ba-46-X; X = Si100Al92O384) have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21 degrees C (a = 24.671(5), 25.024(4), and 25.266(5) Angstrom, respectively). Their structures were refined to the final error indices R-w = 0.046 with 439 reflections, 0.037 with 434 reflections, and 0.049 with 485 reflections, respectively, for which I > 3 sigma(I). In Mg-46(H2O)(4)-X, Mg2+ ions are found at four crystallographic sites : 14 per unit cell are located at site I at the center of the hexagonal prism (Mg-O 2.262(6) Angstrom), only four are at site I’ in the sodalite cavity near the hexagonal prism (Mg-O 2.221(5) Angstrom), four, each coordinated to an H2O molecule, are located at site II’ in the sodalite cavity (Mg-O 2.223(12) Angstrom), and the remaining 24 are at site II near single six-oxygen rings in the supercage (Mg-O = 2.184(5) Angstrom). In Ca-46-X, Ca2+ ions are found at only two sites : 16 per unit cell fill site I (Ca-O = 2.429(8) Angstrom) and the remaining 30 at site II (Ca-O = 2.276(5) Angstrom). In Ba-46-X, Ba2+ ions are again found at three sites : 14.5 per unit cell are at site I (Ba-O = 2.778(11) Angstrom), only 1.5 are at site I’ (Ba-O = 2.44(3) Angstrom), and 30 are at site II (Ba-O = 2.667(8)Angstrom). In the three crystals, sites I and II are the most populated; sites I’ and II’, if they were used, are only sparsely occupied. Ca2+ appears to fit the octahedral site I best. No cations are found at sites III or III’, all of which are clearly less favorable for alkaline-earth cations in dehydrated zeolite X.