The crystal structure of an ethylene sorption complex of fully vacuum-dehydrated fully Ag+-exchanged zeolite X (FAU), a = 24.865(2) (A) over circle, has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 degrees C. It is very different from the ethylene complex of Ag-92-X that had been dehydrated at 400 degrees C in flowing oxygen, as were the two dehydrated structures. The crystal was prepared by ion exchange in a flowing stream of aqueous 0.05 M AgNO3 for 3 days, followed by dehydration at 400 degrees C and 2 x 10(-6) Torr for 2 days, followed by exposure to 300 Torr of zeolitically dry ethylene gas for 2 h at 21 degrees C. The structure was determined in this atmosphere and was refined using all data to the final error indices (based upon the 534 reflections for which F-o > 4 sigma(F-o)) R-1 = 0.062 and wR(2) = 0.135. In this structure, per unit cell, 14 Ag+ ions were found at the octahedral site I (Ag-O = 2.611(9) (A) over circle), and 32 partially reduced Ag+ ions fill two different site I' positions deep in the socialite cavities (Ag-O = 2.601(13) and 2.618(12) (A) over circle). The sodalite cavities host two different cationic silver clusters. In about 47% of socialite units, eight silver atoms form interpenetrating tetrahedra, Ag-8(n+) (n = 4 is suggested), with T-d symmetry. The other 53% of the socialite units host cyclo-Ag-4(m+) (m = 2 is suggested) cations with near S-4 symmetry. These clusters are very similar to those in vacuum-dehydrated Ag-92-X. Thirty-two Ag+ ions fill the single 6-rings, 15 at site II' (Ag-O = 2.492(10) (A) over circle), and 17 at site II (Ag-O = 2.460(9) (A) over circle). The latter 17 lie in supercages where each forms a lateral pi-complex with an ethylene molecule. In turn, each C2H4 molecule forms two cis electrostatic hydrogen bonds to framework oxygens. The remaining 14 Ag+ ions occupy three different II' sites. Vacuum dehydration had caused substantial decomposition: per unit cell, 30 of the 92 Ag+ ions were reduced and 15 of the 384 framework oxide ions were oxidized to O2(g), leaving lattice vacancies. The sorption of C2H4 at 21 degrees C reoxidized about 7 of the 30 Ag-o atoms to Ag+ and reduced 1.75 ethylene molecules to give CH22- groups which refilled 3.5 of these 15 lattice vacancies. The remaining vacancies may have been filled with H2C=C2- ions. The unit cell formula, which originally contained 384 oxygen atoms, may be /Ag-92(C2H4)(17)/ [Si100Al92O369(CH2)(3.5)] or /Ag92H23(C2H4)(17)/[Si100Al92O369(CH2)(3.5)(C2H2)(11.5)].