Octadecasil, a clathrate-type inclusion compound, has been synthesized hydrothermally at 453 K with a gel having the composition 1.0SiO(2):0.53tetramethylammonium (TMA(+)):0.54fluoride:86H(2)O. The crystal structure has been determined based on powder X-ray diffraction data taken at 298 K, and has been refined using Rietveld method. The result confirms the AST-type, all-silica framework model developed by Caullet et al. [P. Caullet, J.L. Guth, J. Hazm, J.M. Lamblin, H. Gies, Eur. J. Solid State Inorg. Chem. 28 (1991) 345]. Furthermore, by using a rigid body model the position and orientation of the occluded TMA(+) cation in the rhombododecahedral [4(6)6(12)] cage can be determined; F- anion has been located in the hexahedral [4(6)] cage. The unit cell parameters, in the tetragonal space group I4/m, have been refined as: a = b = 9.07 angstrom, c = 13.44 angstrom, cell volume = 1104.97 angstrom(3). The refined unit cell composition is \[N(CH3)(4)(+)](2.0)F-1.9(-)\[Si20O40], i.e., both TMA(+) and F- ions possess near full occupancies, and compensate each other's electronic charges. The crystallization of the AST framework structure is the result of a cooperative structure-directing effect of both ions. (C) 2005 Elsevier Ltd. All rights reserved.