The polyatomic tetrahydroxytetraindium(III) cations, In4(OH)48+, have been synthesized in 94% of the sodalite cavities of zeolite Y (FAU, Si/Al = 1.69). In−Y (|In57.8[In5]1.8|[Si121Al71O384]−FAU or In66.8−Y) was prepared by the oxidative solid state ion exchange (OSSIE) method from In0 and single crystals of Tl−Y under anhydrous conditions at 623 K. One of these was exposed to the atmosphere and then zeolitically dry oxygen, both at 623 K, to give In−Y[In4(OH)4] (|In7.6[In4(OH)4]7.5[In(OH)2]2.0|[Si121Al71O384]−FAU or In39.6(OH)34.0−Y). The structures of In−Y and In−Y[In4(OH)4] were determined by single crystal crystallography with synchrotron X-radiation. They were refined in the space group Fd3m (a = 24.863(1) and 24.518(1) A) with all unique data to the final error indices R1 = 0.050 and 0.057 for the 1110 and 1037 reflections for which Fo > 4σ(Fo), respectively. The In in In−Y is mostly In+; some In57+ clusters are present in sodalite cavities. Upon oxidation in the presence of water, 41% of the In ex...