The structures of the subnanometer clusters of CdS that form in zeolite Y (FAU), prepared as follows, have been determined. A single crystal of Cd 2+ -exchanged zeolite Y was prepared by the exchange of Na 75 −Y (|Na 75 |[Si 117 Al 75 O 384]−Y with aqueous Cd(NO 3 ) 2 at 294 K, followed by vacuum dehydration at 723 K (crystal 1). A second crystal, similarly prepared, was exposed to 6.7 × 10 4 Pa of dry H 2 S(g) for 6 h at 294 K and evacuated (crystal 2). Their structures were determined crystallographically using synchrotron X-rays and were refined using all data to the final error indices (calculated using only0.040/0.156 and 0.049/0.186, respectively. In crystal 1, Cd 2+ ions primarily occupy sites I and II. In crystal 2, |Cd 27 (Cd 4 S 6+ ) 0.6 (Cd-(SHCd) 4 6+ ) 0.9 H 12 |[Si 117 Al 75 O 384 ]−FAU, tetrahedral Cd 4 S 6+ ions center about 7.5% of the sodalite cavities and tetrahedral Cd(SHCd) 4 6+ ions center about 11% of the supercages. In Cd(SHCd) 4 6+, a Cd 2+ ion at the center of a supercage bonds tetrahedrally to four SH − ions, each of which bonds in turn to an 8-ring Cd 2+ ion. Of the 36.5(9) Cd 2+ ions per unit cell (indicating complete or nearly complete Cd 2+ exchange) before treatment with H 2 S(g), only 6.9 participate in the above clusters. Most, 27, remain uncomplexed, and 2.6 have left the zeolite as CdS(s). No neutral clusters of formula Cd n S n are present within the zeolite.