Poly(ethyl methacrylate-co-acrylonitrile) (PEAMN20) nanocomposites were successfully prepared via free radical in situ polymerization using a bentonite from Algeria modified by Hexadecyltrimethylammonium chloride (HDTMA). X-ray diffraction (XRD) and transmission electronic microscopy (TEM) investigations revealed that depending on the OMMT loading, intercalated or partially exfoliated nanocomposites were obtained. These nanocomposites exhibited an overall improved thermal stability and an increase in their glass transition temperature compared to the pure copolymer (PEMAN20) as evidenced by Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) analysis. Apparent activation energies (E-alpha) of thermal decomposition of the elaborated nanocomposites, determined by Tang method, were higher than those of the virgin copolymer. The significant increase of (E-alpha), observed with PEMAN20/OMMT (1 wt%) confirmed the change in the degradation mechanism with OMMT loading and the ratio of intercalated/exfoliated structures. (C) 2013 Elsevier B.V. All rights reserved.