In this work, a detailed investigation of the rheological and mechanical properties of nanocomposites based on polyethylene tereplithalate (PET) and montmorillonite clays is presented. A series of additives [maleic anhydride (MAH), pentaerythritol (PENTA), and alkylammonium chlorides from amines of various chain lengths] were used as compatibilizers. The influence of the additives on the mechanical and rheological behavior of the PET matrix was evaluated separately, through their individual contributions. To shed more light on the reported decrease in melt viscosity of organoclay composites with respect to the matrix viscosity, observed in nylon and PET nanocomposites, the polymer molecular weight was related to the resulting rheological and mechanical properties of the systems. Results reveal that the PET nanocomposites behave quite differently in shear as opposed to elongation. The viscoelastic proper-ties in shear present low values when the molecular weight decreases because of the processing operations. However, properties under tension increase over those of the matrix, manifesting the presence of a network formed between the clay and the polymer. (C) 2004 Society of Plastics Engineers.