Composites of carbon nanotubes (CNT) and epoxidized natural rubber (ENR) were prepared by in-situ functionalization of CNT with two alternative silane coupling agents: bis(triethoxysilylpropyl) tetrasulfide (TESPT) and 3-aminopropyltriethoxysilane (APTES). The reactions of ENR molecules with the functional groups on CNT surfaces and with the silane molecules were characterized by Fourier transform infrared. Furthermore, cross-link density, relaxation behaviors, curing, mechanical, electrical, and morphological properties of pristine ENR and the ENR composites were investigated. Very low percolation thresholds, at CNT concentrations as low as 1 phr, were observed in the ENR-CNT and the ENR-CNT-TESPT composites. This might be attributed to improvements in the chemical linkages between ENR molecules and functional groups on CNT surfaces that led to a homogenous dispersion of CNTs in the ENR matrix, with loose CNT agglomerates. POLYM. ENG. SCI., 55:2500-2510, 2015. (c) 2015 Society of Plastics Engineers