Polystyrene/montmorillinite nanocomposites were prepared via in situ bulk polymerization. New imidazolium salts were used to modify montmorillonite obtaining an improvement in thermal stability in comparison with a montmorillinite modified with a standard alkylamonium cation. The synthesized cations facilitate the formation of partially exfoliated structures because of the presence in their structure of a polymerizable group. The polystyrene/montmorillonite nanocomposites were characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).