The grafting reaction between a trifunctional silylating agent and two kinds of 2:1 type layered silicates was studied using FTIR, XRD, TGA, and Si-29 CP/MAS NMR. XRD patterns clearly indicate the introduction of 3-aminopropyltriethoxysilane (gamma-APS) into the clay interlayer. In the natural montmorillonitc, gamma-APS adopts a parallel-bilayer arrangement, while it adopts a parallel-monolayer arrangement in the synthetic fluorohectorite. These different silane arrangements have a prominent effect oil the mechanism of the condensation reaction within the clay gallery. In natural montmorillonite, the parallel-bilayer arrangement of gamma-APS results in bidentate (T-2) and tridendate (T-3) molecular environments, while the parallel-monolayer arrangement leads to monodentate (T-1), as indicated by Si-29 CP/MAS NMR spectra. This study demonstrates that the silylation reaction and the interlayer inicrostructure of the grafting products strongly depend on the original clay materials. (c) 2005 Elsevier Inc. All rights reserved.