A spatially ordered quantum dot array of cationic indium nanoclusters has been synthesized in a single crystal of zeolite X. A single crystal of fully dehydrated and fully indium-exchanged zeolite X (ca. In 87 Si 100 Al 92 O 384 , In 87 -X) was exposed to 0.5 atm of H 2 S for 12 h at 673 K. After it was evacuated at temperature and cooled to 294 K, the crystal structure of the product (ca. In 66 Si 100 Al 92 O 384 , In 66 -X, a ) 24.942(4) Å) was determined by single crystal X-ray diffraction techniques in the cubic space group Fd3 hm. It was refined with all measured reflections to the final error index R 1 ) 0.058 for the 754 reflections with F o > 4σ(F o ). X-ray photoelectron spectroscopy experiments confirmed the existence of both atomic and cationic indiums inside the single crystal. The 66 indium atoms or ions per unit cell were found at four crystallographically distinct positions, all on 3-fold axes. Eight In atoms at the sodalite unit centers and 32 In ca.2+ ions nearby (In ca.2+ -In 0 ) 2.683(1) Å), both filling their crystallographic sites, form eight (In 5 ) n+ (n ) 7 satisfies Lewis's rule) per unit cell, one per sodalite cavity. Each (In 5 ) n+ quantum dot (diameter ca. 7.0 Å) is held in place by 12 strong electrostatic interactions with six ring oxygens (In ca.2+ -O(3) ) 2.170(7) Å). These quantum dots are arranged in the diamond structure with intercluster distances of ca. 10.8 Å. This crystal (cross-section ca. 0.15 mm) contains ca. 300 Tera (10 12 ) ordered dots of (In 5 ) n+ . Another 25 In + ions per unit cell were located at a 32-fold site in the supercage, and a single In 2+ ion was found nearby. Treatment with H 2 S has allowed In + ions to disproportionate to give full occupancy at the (In 5 ) n+ site.