In this work, nanocomposites of poly (m-xylene diamide) (MXD6) with dodecaphenyl polyhedral oligomeric silsesquioxane (POSS), montmorillonite (MMT) and their combination (MMT-POSS) were produced by melt-compounding and their structureproperty relationships were established. The montmorillonite was found to be homogeneously dispersed affording intercalated disordered structures whilst the distribution and dispersion of the POSS was rather skewed with the presence of phase separated crystalline aggregates of up to several hundred nanometers. Significant differences in properties were observed depending on the type of nanoscale filler present in the nanocomposites. The montmorillonite provided some improvements in stiffness and dynamic response of MXD6. POSS on the other hand improved the ductility of MXD6 and had a plasticizing effect. It has been suggested that there might be a lubricating action at the interface inducing better molecular motion between polymer chains of MXD6. By combining MMT and POSS, the properties of the corresponding nanocomposites were found to be intermediate. The thermal stability of MXD6 was enhanced in the nanocomposites although a negative deviation from the theoretical model was observed when POSS was present in the nanocomposites. This observation underlies some antagonistic effects which have been attributed to the presence of catalytic benzene radicals during thermal degradation. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers