The aim of this work is to characterize the rheological and permeability behavior of nanocomposites based on amorphous poly(ethylene terephthalate) (PETg) and organically modified montmorillonites (omMMT), obtained by melt intercalation. The use of PETg instead of semicrystalline PET is believed to reduce the risks associated to organic modifier degradation during processing at high temperatures. X-ray and transmission electron microscopy analysis performed on the PETg nanocomposites showed that processing for long time at temperatures lower than melting of semicrystalline PET allowed to obtain a partially intercalated structure with some degree of exfoliation. The rheological behavior of PETg nanocomposites was studied as a function of shear rate in a cone-plate rheometer in order to correlate the viscosity with the aggregation state of omMMT. A simple model accounting for an apparent increase of rheological units size, associated with the intercalation of PETg macromolecules into omMMT galleries, is proposed. The glass transition temperature, T, as a function of the volume fraction of omMMT content of the nanocomposite, was measured using differential scanning calorimetry. Finally, the water vapour permeability of PETg nanocomposites was correlated to the volume fraction of the impermeable inorganic part of the omMMT. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 3666-3672, 2010