The vinyltrimethoxysilane-grafted ethylene-propylene copolymer/trifunctional methoxysilane (EPR-g-VTMS/RTMS) composites were prepared via in situ silica sol-gel reactions. Five trifunctional methoxysilane compounds (n-hexyltrimethoxysilane, n-decyltrimethoxysilane, n-tetradecyltrimethoxysilane, n-octadecyltrimethoxysilane, and phenyltrimethoxysilane) have been selected for this study. The water-cross-linked EPR-g-VTMS/RTMS composites were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy, gel content, solid-state Si-29 CP/MAS NMR, wide-angle x-ray scattering, tensile strength, and field emission scanning electron microscopy measurements. The type of RTMS additive has a substantial influence on the nature of siloxane band networks and eventually the mechanical tensile properties. This finding suggests that the interaction and/or entanglement between the EPR-g-VTMS matrix and the substituent of the RTMS additives are crucial for the modifying mechanical properties. Moreover, for the water-crosslinked EPR-g-VTMS/CnTMS (n = 6, 10, 14, and 18) composites, the joint evidence provided by attenuated total reflectance-Fourier transform infrared spectroscopy, 29Si CP/MAS NMR, and wide-angle x-ray scattering results suggested the formation of ladder-type poly(n-alkyl silsesquioxane)s and the presence of the highly ordered structure with a thickness equal to the length of two n-alkyl groups in all-trans conformation. POLYM. ENG. SCI., 51:632-640, 2011. (C) 2010 Society of Plastics Engineers