The crystal structure of a mesitylene sorption complex of fully dehydrated Ca2+-exchanged zeolite X, Ca46Si100Al92O384.8C(9)H(12) (a = 24.966(4) Angstrom), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 degreesC. The crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Ca(NO3)(2) for 4 d, followed by dehydration at 400 degreesC and 2 x 10(-6) Torr for 2 d (colorless) and then exposure to ca. 2.1 Torr of zeolitically dry mesitylene vapor at 20 degreesC. The structure of the resulting dark-brown crystal was determined in this atmosphere and was refined to the final error index R-1 = 0.046 with 253 reflections for which F-o > 4sigma(F-o); wR(2) based on F-2 and all data (1153) is 0.128. In this structure, 16 Ca2+ ions fill site I (at the centers of the double 6-rings), and the remaining 30 Ca2+ ions are found at two nonequivalent sites II (near 6-rings in the supercages) with occupancies of 22 and 8 ions. Each of these Ca2+ ions coordinates to three framework oxygens and extends 0.262 or 0.584 Angstrom, respectively, into the supercage from the plane of the three oxygens to which it is bound. One mesitylene molecule per supercage lies on a 3-fold axis where it interacts facially with one of the latter eight Ca2+ ions (Ca2+-mesitylene center = 2.70 Angstrom). All three hydrogen atoms of the central ring and one hydrogen atom of each methyl group are near framework oxygens. (Hydrogen atom positions were calculated from carbon positions.) Altogether, each mesitylene molecule has six close H...O interactions with zeolite oxygens: three ring H...O = 2.70 Angstrom and three methyl H...O = 2.52 Angstrom. Mesitylene appears to be distorted in the same way and to a similar extent as half of the strained molecule [2.2.2](1,3,5)cyclophane-1,9,17-triene.