The preparation and physical properties of a series of nanocomposites based on dispersions of Montmorillonite clays in thermoset epoxy resins are reported. The effects of the variation of the concentration of the clay and the influence of a change of the functionality of the epoxy compounds and the amine curing agent are reported. The effects of the method of dispersion of the clay are studied, and it was found that ultrasound provides an effective aid to dispersion of the clay platelets. In general, the addition of clay platelets leads to an increase in the glass-rubber transition, but in the case of a highly cross-linked system, the reverse effect was observed. The effects observed are discussed in the context of the way in which the chemical structure of the monomers influence the dispersion process and the structure of the final resin system.