The phenyltrimethylammonium and benzyltrimethylammonium silicate hydrates with the approximate chemical compositions [NPhMe 3] 6[Si 8O 18(OH) 2]·38.7H 2O ( 1) and [NBnMe 3] 8[Si 8O 20]·53.6H 2O (Ph=phenyl, Me=methyl, Bn=benzyl) ( 2) have been prepared, and their crystal structures were determined by single-crystal X-ray diffraction methods. Crystal data: 1: monoclinic, P2 1/ n, Z = 4, a = 23.70(1) Å, b = 15.733(5) Å, c = 28.49(1) Å, β = 93.72(4)°, T = 225 K; 2: triclinic, P 1 , Z = 2, a = 15.48(1) Å, b = 18.594(6) Å, c = 25.413(5) Å, α = 78.42(3)°, β = 87.03(4)°, γ = 86.74(7)°, T = 175 K. Compounds 1 and 2 are host-guest compounds with mixed three-dimensional host structures composed of cube-shaped octameric silicate anions and water molecules linked via hydrogen bonds OH…O (heteronetwork clathrates). The microchannels in 1 and 2, which open up into rather large cavities in 2, are occupied by the quaternary ammonium cations, [NPhMe 3] + or [NBnMe 3] +, as guest species. The cube-shaped oligomeric [Si 8O 18(OH) 2] 6− and [Si 8O 20] 8− anions are stabilized within very similar local organic-water surroundings by OH…O bonds as well as electrostatic and Van der Waals O…CH 3(N) and O…CH 2(N) anion-cation interactions. Some features of the crystal structures are of considerable interest with regard to a recently proposed mechanism for the formation of mesostructured silicate-surfactant composite materials (M41S) in alkaline solution.