ABSTRACT Nanocomposites are new materials submitted to many researches since about ten years. In these organic /mineral nanocomposites, ultrafine dispersion and local interactions between polymer and reinforcing agent lead to better properties than those of micro or macrocomposites., even for low fraction of reinforcing agents. A lot of researches have been soon conducted on Polymer Layered Silicate (PLS) synthesis, according to the polymer nature (polyamide, polystyrene, polypropylene, epoxy .) Recently, Wang et al (1) carries out studies on PVC-organoclay and PVC-sodiumclay systems. The composites obtained have intercalated and partially exfoliated structures. Telica et al (2) have also done researches on such systems. In all these studies, the greatest problem lies in the thermal decomposition of PVC. At high temperature, PVC alone is not thermally stable. Moreover, PVCorganoclay system is less thermostable than PVC alone. Nevertheless, the mechanism of decay of such systems has not yet been entirely explained. In this work, the influence of quaternary ammonium salt on the decomposition of PVC-organically modified montmorillonite (MMT) blends is studied. According to literature, three assumptions are proposed and explored to explain the decomposition of PVC by ammonium. Evolution of torque on brabender plasticorder, HPLC-MS and RMN spectrum have been used to propose a mechanism for the degradation of PVC-ammonium quaternary modified MMT.