Different kinds of clays based on sodium montmorillonite have been modified by (1) cationic exchange of alkylammonium ions and (2) covalent grafting of organosilane. We have prepared organophilic clays with different gallery heights (due to alkylammonium ions with alkyl chain length varying from 8 to 18 carbon atoms) and with a specific functionalization (due to the organosilane). The experimental parameters of the cationic exchange process (exchange temperature, initial amine/clay ratio, washing of the clay) govern the quantity of the intercalated alkylammonium ions as well as their organization in the montmorillonite galleries. We have generated organoclays with different kinds of organic layer properties, namely ionically bonded or physically adsorbed alkylammonium ions and covalently grafted organosilane. These different surface treatments will be useful for different applications, varying as a function of the coupling with the matrix and as a function of filler dispersion and resulting properties (thermal stability, mechanical and barrier properties) of the final nanocomposite.