Single crystals of zeolite Y, |Na 71 |[Si 121 Al 71 O 384 ]-FAU, with diameters up to 0.20 mm were grown. They were ion exchanged to generate |(NH 4 ) 64 Na 5 K 2 |[Si 121 Al 71 O 384 ]-FAU. Crystals 1 through 6, respectively, were vacuum dehydrated at 323, 373, 423, 473, 523, and 573 K, and without reexposure to the atmosphere, their structures were determined crystallographically at 294 K using synchrotron X-radiation. Their compositions (best integers) are seen to be respectively |(NH 4 ) 59 H 5 Na 5 K 2 (H 2 O) 3 |[Si 121 Al 71 O 384 ]-FAU (incomplete dehydration and partial deamination), |(NH 4 ) 31 H 33 Na 5 K 2 |[Si 121 Al 71 O 384 ]-FAU (further deamination), |(NH 4 ) 2 H 62 Na 5 K 2 |[Si 121 -Al 71 O 384 ]-FAU (nearly complete deamination), |H 61 Na 5 K 2 (Al 2 O) 0.5 |[Si 122 Al 70 O 384 ]-FAU (onset of dealumination and framework reconstruction), |H 53 Na 5 K 2 (Al 2 O) 2 |[Si 124 Al 68 O 384 ]-FAU (further dealumination and reconstruction), and |H 51 Na 5 K 2 (Al 2 O) 2.5 |[Si 124 Al 68 O 384 ]-FAU. The extent of deamination of NH 4 + increases with increasing evacuation temperature until it is complete in crystal 4. In crystal 1, 59 NH 4 + ions per unit cell are found at two crystallographically distinct positions: 30 at site I′ (in sodalite cavities opposite D6Rs) and 29 at site II (in supercages opposite S6Rs). Three water molecules coordinate to Na + ions. In crystal 2, 17 NH 4 + ions per unit cell are found at site I′ and 13 at site II. In crystal 3, only 2 NH 4 + ions at site II are found. The extent of dealumination of the zeolite framework, seen in crystals 4, 5, and 6, also increases with temperature: 1, 4, and 5 Al 3+ ions per unit cell, respectively, are found at site I′ recessed slightly into D6Rs. Each Al 3+ ion coordinates trigonally (O-Al-O ) 119.3°) at ca. 1.85 Å to three framework oxygens. Pairs of these Al 3+ ions are each bridged by a nonframework oxide ion at the center of a D6R to give linear Al-O-Al groups (Al-O ) 1.62 Å). These Al 3+ ions are Lewis-acid sites and should be catalytically active when accessible. Five additional crystals vacuum dehydrated at 623 K and above showed no crystallinity, due presumably to further dealumination and dehydration (loss of H + and framework oxygen).