This work investigated the mechanical behavior of POSS-modified polyamide 6 (PA6), containing PA6 chains terminated at one end with aminopropyl-heptaisobutyl FOSS. Three systems with different POSS contents (5.4, 10.8, and 16.2 wt %), each characterized by a specific molecular mass, and the corresponding PA6 neat polymers were examined. The materials were first subjected to morphological and calorimetric analysis by wide-angle X-ray diffraction and differential scanning calorimetry, respectively. Tensile tests, performed on the samples in wet conditions, showed that stiffness, strength, and ductility were appreciably modified by the presence of POSS in the polymeric chains. The relationships among these effects and the microstructural characteristics of the systems also were analyzed. (c) 2006 Wiley Periodicals, Inc.