The product of the reaction of HgCl2 vapor with fully dehydrated fully NH4+-exchanged zeolite X at 115 degrees C, ca. Hg44Cl64(NH4)(68)-Si100Al92O384 per unit cell, was studied by single-crystal X-ray diffraction techniques at 23 degrees C (R-1 = 0.121) in the cubic space group Fd (3) over bar (a(0) = 25.094(8) Angstrom). Hg2+ ions are found at seven nonequivalent sites, a surprisingly large number. Hg2+ ions at sites T and II each coordinate only to framework oxygens; this is the first observation of Hg2+ ions at conventional ionic sites in a zeolite, and of very nearly regular octahedral Hg(II), and of three-coordinate trigonal Hg(II). Hg2+ ions at two general site III＇ positions form (HgCl2)(3)(NH4+)(2) and (HgCl2)(3)(NH4+) clusters, respectively, in 12-rings with each Hg2+ ion bonding to one framework oxygen. The Hg2+ ions at another two general site III＇ positions form (HgCl2)(6)(NH4+)(2) clusters, also in 12-rings with each Hg2+ ion bonding to two framework oxygens. These two clusters could bridge through Cl- ions to form a supercluster that contains as alternative subunits Hg12Cl12 rings each of which wraps around a double six-ring. Site II is full with two Hg2+ and 30 NH4+ ions. In each sodalite cage, an NH4+ ion at site I＇ bonds to three framework oxygens. Two Hg2+ ions per unit cell are seen near supercage centers; each coordinates to three Cl- ions that bridge to other Hg2+ ions. The desired reaction of NH4-X with HgCl2 vapor did not go to completion. Substantial amounts of HgCl2 were sorbed and cationic superclusters of Hg(II) and Cl- have formed. This is the first observation of cationic halomercuric species.