In this work, poly(ethylene terephthalate)/organically modified montmorillonite (PET/o-MMT) nanocomposites were prepared via direct melt compounding in a twin-screw extruder. The main objective was to study the effects of using a polyester ionomer (PETi) as a compatibilizer to promote the intercalation and/or exfoliation of the o-MMT in the PET. The o-MMT content was 0, 1, 3, or 5 wt % and the PETi/o-MMT mass ratio was 0/1, 1/1, or 3/1. The PETi was efficient to promote the intercalation/exfoliation of the o-MMT in the PET matrix, as revealed by wide angle X-ray scattering and transmission electron microscopy. Rheological characterization showed that the PET/o-MMT nanocomposites exhibited a higher complex viscosity at low frequencies than PET, which is characteristic of materials presenting yield strength. Moreover, the higher the content and/or the degree of intercalation/exfoliation of the o-MMT, the more the nanocomposite behaved like a solid because of a percolated structure formed by the o-MMT layers, and the more the storage and loss modulus, G' and G '', became independent of the frequency at low frequencies. (c) 2007 Wiley Periodicals, Inc.