Blends of poly(etheretherketone) (PEEK) and poly(etherimide) (PEI) were prepared directly during the plasticization step of an injection-molding process. The phase behavior and mechanical properties of the blends at both 23° and 125°C were studied. On the basis of differential scanning calorimetry (DSC) and dynamic-mechanical analysis (DMTA), all the blends exhibited a single amorphous phase. Both the as-molded and annealed blends showed at 23°C an overall synergistic behavior in ductility and in impact strength. The different crystalline levels influenced the properties but when there was no crystallinity effect, the intrinsic compatibility of PEEK and PEI was also clear and appeared as the probable main reason for the observed mechanical behavior. At 125°C the modulus of elasticity and tensile strength of annealed blends and the modulus of elasticity of as-molded blends were below the additive values. However, in the as-molded blends, the tensile strength and, mainly, the ductility, were synergistic.