In the present study, nanocomposites were prepared with different contents (1, 2, 5, 10% by weight) of polyhedral oligomeric silsesquioxane—POSS n‐phenylaminopropyl in an epoxy matrix. The samples were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and X‐ray diffraction (XRD). TGA analyses were performed at different heating rates to allow for the prediction of the kinetic parameters of degradation. XRD analyses were used to calculate the average molecular interchain spacing. TGA analysis showed that the presence of POSS increased the thermal stability of the nanocomposites. An increase in the activation energy on degradation was evidenced through the kinetic parameters, especially for the 10% POSS. Moreover, the addition of POSS changed the kinetic mechanism of the deceleratory mode (Fn) in favor of the diffusion (Dn) process. The diffusion mechanism is corroborated by the XRD analyses that showed the increase of distance between chains when was added POSS. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers