In this work we report the properties of nanocomposite based on PET with two different samples of organically modified montmorillonites. In particular, we studied the effect of the filler concentration on morphology, rheology, and mechanical performance, focusing our attention on the effect of the degradation phenomena of the clay modifiers. The results indicate that at low clay level the morphology achieved is mainly intercalated. On increasing the filler level, coalescence and/or bad defragmentation phenomena induce a coarser morphology, as confirmed by XRD, SEM, and TEM observations. When a more polar organic modifier is used to modify the clay, the particle adhesion and distribution is slightly better. Conversely, at the processing temperatures adopted, this organic modifier induces a strong degradation of PET, as confirmed by melt rheology and intrinsic viscosity measurements. DSC indicates, in addition, a slight increase of crystallinity likely due to the decreased molecular weight. As regards the mechanical properties, Young's modulus is not significantly changed unless high amounts of clay (10%) are used while the elongation at break drops even at the lowest clay content. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 384-392, 2011