The reinforcing effect of organoclay in two epoxy matrices, one rubbery and one glassy, was studied. The rubbery and glassy epoxy matrices were chosen to have a very similar chemistry to minimize its impact on the comparison of properties. The epoxy resin was EPONTM 828, and the two hardeners were amine-terminated polyoxypropylene diols, having different average molecular weights (MW) of 2000 and 230 g/mol, namely Jeffamine (R) D-2000 and Jeffamine (R) D-230, respectively. The nanocomposites were prepared with the organoclay Cloisite (R)) 30B from Southern Clay Products. The quality of dispersion and intercalation/exfoliation was analyzed by means of X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEGSEM), and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the curing reactivity and the thermal stability of the epoxy resin systems, respectively. Tensile properties and hardness of epoxy resin and epoxy nanocomposites were measured according to ASTM standards D638-02 and D2240-00, respectively. Fracture surfaces were also analyzed by FEGSEM. These two epoxy systems as well as their nanocomposites display totally different physical and mechanical behavior. It is found that the quality of clay dispersion and intercalation/exfoliation, and the mechanical behavior of the glassy and rubbery epoxy nanocomposites are distinct. The results also indicate that the presence of the clay does not significantly affect the T-g of either the rubbery or the glassy epoxy; however, the fracture surface and mechanical properties were found to be influenced by the presence of nanoclay. Finally, several different reinforcing mechanisms are proposed and discussed for the rubbery and glassy epoxy nanocomposites. (C) 2007 Wiley Periodicals, Inc.