A study on the mechanisms for embrittlement of polyamide-6 (PA-6) by nanometer-sized clay in nanocomposites is reported in this paper. Tensile modulus and yield strength in these composites were found to increase with clay concentration, while the strain at rupture decreased. Similar to the strain at rupture, fracture toughness also decreased dramatically. Investigation by transmission electron microscopy alone indicates that multiple crazing appears to be the only significant toughening mechanism for the nanocomposite with 2.5 wt % clay which does not explain why composites with more clay are significantly more brittle. The deformation behavior was therefore further studied using wide (WAXS) and small (SAXS) angle X-ray scattering, scanning electron microscopy (SEM), and optical microscopy. WAXS detected the existence of the γ-phase of PA-6, probably due to the presence of clay. However, the occurrence of this crystalline phase cannot explain the observed changes in mechanical behavior. SAXS study indica...