In situ reinforced binary and ternary polymer/polymer composites are obtained by the melt blending of poly(ethylene terephthalate) (PET), polyamide-6 (PA-6), and polyamide-66 (PA-66) in an extruder in the presence of a catalyst, followed by drawing of the extrudate and annealing of the drawn blends. The blends were studied by DSC, X-ray, SEM, and mechanical testing. After drawing, all the components were found to be oriented, forming microfibrils with diameters of about 1-2 mu m. The chemical nature of the homopolymers affects the blends＇ morphologies; while the PA-66/PA-6 blend is homogeneous, phase separation is established in the case of PET/PA-6. The decrease in the enthalpy of melting of the blend components as well as the depression of their peaks of crystallization from the melt, compared to pure homopolymers, are indications that block copolymers have been formed via interchange reactions during the blending process. On the one hand, these copolymers improve the compatibility of the homopolymers, and on the other hand, they alter the chemical composition of the blends. After thermal treatment at 245 degrees C, i.e., above the T-m of PA-6, the latter undergoes some disorientation, while PET and PA-66 retain their microfibrillar shape, and in this way, a compositelike structure is created. The presence of chemical bonds between the separate phases via copolymers favors the cocrystallization of PA-66 and PA-6 as well as the cooperative crystallization of PET, PA-6, and PA-66, both modes fostering improved compatibility (adhesion) of the blend components.