The long range objective of this research is to understand to what extent the presence of nanosized particles may change the local properties, and specifically perm selectivity characteristics, of a glassy polymer matrix or whether conventional composite theory can be applied to such composites, i.e., the matrix properties are not changed by the filler. In this work, nanocomposite membranes based on an amorphous, glassy poly(ether imide) were formed by incorporating three kinds of hydrophobically treated fumed silica by solution casting and melt processing techniques. However, there is considerable evidence that these nanocomposites contain voids or defects, probably at the polymer-particle interface or within aggregates, that increase gas permeability and decrease selectivity. Thus, the primary focus of this paper is the relation between the extent of voids formed in the nanocomposite and the permeation properties. In addition, this paper deals with techniques for dispersing nanosized particles, fumed silica, in the polymer matrix, characterization of morphology of this mixture using TEM and SEM, and evaluation of local properties based on gas permeation. (c) 2006 Elsevier Ltd. All rights reserved.