Poly(ethylene terephthalate) (PET) based nanocomposites containing 3 wt % of different nanoparticles (MontMorilloniTe-MMT; titanium dioxide-TiO2; and silica dioxide-SiO2) were prepared via two independent procedures: mechanical mixing with subsequent direct injection molding (DIM) and mechanical mixing, followed by extrusion blending and injection molding (EIM). The contributions of nanofillers with respect to pure PET were evaluated. The incorporation of nanofillers reduces the intrinsic viscosity of the polymer matrix when processed by DIM and EIM. SAXS results showed that: MMT layers were intercalated for both processing procedures, but slightly higher for EIM; a better dispersion with smaller agglomerates size is achieved for TiO2 and SiO2 nanoparticles for EIM than for DIM. According to the results of DSC analysis, all fillers behave as nucleating agents for PET except SiO2 that acts as inhibitor in case of DIM procedure. The mechanical behavior was assessed in tensile testing. The mechanical test revealed that the addition of nanoparticles have a slight influence on the elastic modulus and yield stress, but a drastic negative influence on the deformation capabilities of the moldings. The measured optical properties of the moldings gloss and haze are also strongly affected by the presence of nanoparticles. (C) 2007 Wiley Periodicals, Inc.