The crystal structure of an ethylene sorption complex of anhydrous fully Cd2(+)-exchanged zeolite X, Cd-46-Si100Al92O384. 29.5C(2)H(4) (a = 24.864(6) Angstrom, has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd(3) over bar at 21(1)degrees C. A crystal of Cd-46-X was dehydrated at 450 degrees C and 2 x 10(-6) Torr for 2 days followed by exposure to ca. 150 Torr of dry ethylene gas at 21(1)degrees C. The structure was determined in this atmosphere and refined to the final error indices R(1) = 0.046 and R(w) = 0.054 with 740 reflections for which I > 3 sigma(I). At site I (at the centers of the double six-rings), 15.5 octahedrally coordinated Cd2+ ions are found per unit cell (Cd-O = 2.350(6) Angstrom). Only one Cd2+ ion (trigonal) is found at site I’ (Cd-O = 2.503(6) Angstrom). At site II (in the supercage), 29.5 Cd2+ ions are found (Cd-O = 2.221(6) Angstrom). Each of these site-II Cd2+ ions is recessed 0.50(1) Angstrom into the supercage from the plane of the three oxygens to which it is bound. To complete its distorted tetrahedral coordination, each coordinates laterally (symmetrically) to an ethylene molecule (counted as monodentate) in the supercage (Cd-C = 2.70(4) Angstrom and C=C = 1.26(5) Angstrom). No other ethylene molecules are sorbed.