The crystal chemistry of hydronium ions in zeolite X has been investigated by single-crystal X-ray and powder neutron diffraction. By titration of Na96Si96Al96O384 With HCl, a break in the pH was observed at 6.35, at about 33 H3O+ ions per unit cell. The structure of zeolite X in equilibrium with 0.1 M aqueous Na+ at pH 6.35 was determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 20 degrees C after partial dehydration (Na-60(H3O)(32)(H2O)(24)Si100Al92O384, a = 25.128(4) Angstrom) and in Fd (3) over bar m after complete dehydration (Na60H32Si100Al92O384, a = 25.101(2) Angstrom). In the partially dehydrated crystal, Na+ ions are found at three different crystallographic sites and H3O+ ions are found at two. Two different site I＇ positions are occupied by 16 Na+ and 16 H3O+ ions, respectively, per unit cell; a water molecule bridges between two of each in each sodalite cage to give eight near tetrahedral (H3O+)(2)H2O(Na+)(2) clusters. Two different site III＇ positions in the supercage are occupied by 16 Na+ and 16 H3O+ ions, respectively; a water molecule bridges between one of each to give 16 (H3O+)(H2O)(Na+) clusters. Thus a structural basis for the great selectivity of zeolite Na-X for H3O+ at near neutral pHs is demonstrated. The remaining 28 Na+ ions are at site II. In the fully dehydrated single crystal, the 60 Na+ ions are found at four crystallographic sites. Approximately 6 and 20 Na+ ions per unit cell at sites I and I＇, respectively, effectively fill the double six-rings. Thirty-two Na+ ions fill site II and the remaining two are at site III＇. To avoid Na+ ions, at least 20 of the 32 H+ ions per unit cell should be in the supercage. Finally, neutron diffraction of Na-54(D3O)(42)Si96Al96O354. similar to 80D(2)O at 10 K revealed the structures of the 16 D3O+ ions in the sodalite units: D-O = 1.13(1) Angstrom and D-O-D = 88(1)degrees. Each deuteron hydrogen bonds to a framework oxygen atom at 1.53(1) degrees. The O ... O hydrogen-bonding distance is 2.63(1) Angstrom and the O-D ... O angle is 161 degrees.