Extrusion-blended and injection-molded PEI/PBT blends were found to be miscible whatever the composition. The processability of the blends clearly improved with the presence of PET. The melt pressure at the exit was seen to be a parameter as representative of the processability of the blends as the torque of blending. In the blends with 80 and 90% PET, a positive volume of mixing and the maintenance of the crystallinity of PET were seen. However, in the rest of the blends, negative volumes of mixing and important decreases in the crystallinity of PET were found. These solid state features gave rise to a ductility similar to that of the pure PEI and to a synergism of the modulus of elasticity and of the yield stress in the 90/10 and 80/20 blends such that the values were higher than those of either of the pure components.