The present work describes the formation of co-continuous phase morphologies in uncompatibilized and compatibilized poly(ethylene terephthalate) (PET)-poly(m-xylene adipamide) (MXD6) and PET-poly(ethylene-co-vinyl alcohol) (EVOH) melt-extruded blends. Phase continuity has been determined by using the Jordhamo relationship. Viscosity values, which are essential for the calculation of the phase continuity, have been obtained by capillary rheometry. Thermal behavior of the blends has been analyzed by employing differential scanning calorimetry and. phase continuity has been investigated for the noncompatibilized and the compatibilized blends by scanning electron microscopy. PET-MXD6 blends [92.35:7.65 (v/v) and 84.5:15.5 (v/v)] and PET-EVOH blends [73.63:26.37 (v/v)] exhibit droplet-in-matrix phase morphology, whereas uncompatibilized PET-MXD6 [75.8:24.2 (v/v)] blend has a combination of rod-like, droplet-matrix structure, and quasi-interpenetrating network structure. (C) 2009 Wiley Periodicals, Inc. Adv Polym Techn 28: 173-184, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20156