In this study, we use PMMA, tetraethoxysilane and titanium ethoxide to prepare tertiary nanocomposites via non-hydrolytic sol-gel method. Boron trifluoride monoethylamine was used as catalyst. Silica and titanium dioxide are incorporated into nanocomposites to improve the thermal stability. Thermogravimetric analysis was used for rapid evaluation of the thermal stability of different materials. The integral procedural decomposition temperature has been correlated the volatile parts of polymeric materials and used for estimating the inherent thermal stability of polymeric materials. The thermal stability of hybrids increased with the contents of inorganic components. The inorganic components can improve the thermal stability of PMMA copolymer. Two methods have been used to study the degradation of hybrid during thermal analysis. These investigated methods are Kissenger', Ozawa's methods to classify the thermal stability of nanocomposites. The activation energies of hybrids were higher than that of the copolymer. From the results, silica and titania will enhance the thermal stability of PMMA. We use the Solubility test to check the network structure of nanocomposites. The results show the residues of composites after extraction increase with the increasing of inorganic contents. We can conclude that the composites possess network structure. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1959-1965, 2009