Modification of transport properties of an amorphous polyamide, poly (trymethylhexamethylene terephtalamide) by addition of a nanosilicate, montmorillonite has been studied. For this proposal, carbon dioxide, water vapor and oxygen have been investigated in different sorption and permeability devices. Transport properties have also been related to the structural features. X-ray diffraction and transmission electron microscopy have provided the morphological analysis as well as the positron annihilation lifetime spectroscopy has given the free volume characterization. (c) 2007 Elsevier B.V. All rights reserved.