Epoxy/Clay nanocomposites with two organically modified montmorillonites (Cloisite 30B and Cloisite 15A) have been prepared. Cloisite 15A has higher cation exchange capacity, interlayer spancing and hydrofobicity than Cloisite 30B. Different methods were carried out to disperse the clay in the epoxy monomer (diglycidyl ether of bisphenol A) with and without solvent, using stirring and ultrasound sonication. The epoxy hardeners used were 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenylsulfone which generate high glass transition temperature epoxy thermosets. The content of clay in the nanocomposites ranged from 2 to 11 wt%. The effect of Cloisites on the curing reaction has been studied by differential scanning calorimetry, finding that the presence of Cloisite 30B accelerates the curing reaction. The glass transition temperature of the epoxy thermoset decreases when the clay content increases, due to the plasticizing effect of the alkylammonium cations. The dispersion of the layered silicates within the crosslinked epoxy matrix was studied by wide-angle X-ray diffraction. In all the cases, the nanocomposites show intercalated clay structures, being the interlayer clay spacing almost independent of the method of dispersion, of the clay content, and of hardener used. Moreover the d-spacing differences between C30B and C15A nanocomposites are insignificant. Epoxy molecules intercalate in a smaller proportion in C15A than in C30B, as it was deduced from the increase of the d-spacing. The dynamic mechanical thermal properties of these nanocomposites were also investigated. Nanocomposites with Cloisite 30B show higher values of storage modulus than neat epoxy, both in the glassy and in the rubbery states. However Cloisite 15A does not improve the epoxy storage modulus, and such divergent behavior agrees with the different intercalation of epoxy in the clays. The fracture surfaces of the nanocomposites analyzed by environmental scanning electron microscopy indicate an improvement of toughness.