Epoxy-clay nanocomposites were synthesized to examine the effects of the content and type of different clays on the structure and mechanical properties of the nanocomposites. Diglycidyl ether of bisphenol-A (epoxy) was reinforced by 0.5-11 wt % natural (Cloisite Na+) and organically modified (Cloisite 3013) types of montmorillonite. SEM results showed that as the clay content increased, larger agglomerates of clay were present. Nano-composites with Cloisite 30B exhibited better dispersion and a lower degree of agglomeration than nanocomposites with Cloisite Na+. X-ray results indicated that in nanocomposites with 3 wt % Cloisite 30B, d-spacing expanded from 18.4 angstrom (the initial value of the pure clay) to 38.2 angstrom. The glass transition temperature increased from 73 degrees C, in the unfilled epoxy resin, to 83.5 degrees C in the nanocomposite with 9 wt % Cloisite 30B. The tensile strength exhibited a maximum at I wt % modified clay loading. Addition of 0.5 wt % organically modified clay improved the impact strength of the epoxy resin by 137%; in contrast, addition of 0.5 wt % unmodified clay improved the impact strength by 72%. Tensile modulus increased with increasing clay loading in both types of nanocomposites. (c) 2005 Wiley Periodicals, Inc.