Miscibility and an asymmetric T-g-composition relationship have been demonstrated in the binary blends composed of amorphous poly(ether imide) (PEI) and semicrystalline poly(butylene terephthalate) (PET). The peculiar T-g-composition behavior in the miscible PEI/PBT blends was analyzed using a free-volume approach, where uneven contributions to the mixtures free volumes were attributed to the vastly different crystallizing tendency of one constituent versus the other. A single T-g was observed in the PEI/PBT blends using differential scanning calorimetry (DSC) as well as dynamic mechanical analysis (DMA), which proved single relaxation and intimate mixing between these two components at the molecular scales. Additionally, homogeneous phase morphology in the blends was also substantiated using optical and scanning electron microscopy. Interactions between the polymers wore investigated by analyzing the equilibrium melting point depression as well as the T-g dependence on composition.