This present investigation deals with the miscibility, phase behaviour and semicrystalline morphology of new thermoplastic polyimide (N-TPI)/poly(ether imide) (PEI) blends with different compositions. Melt-quenched amorphous blends were used as the raw materials. The phase behaviour of the blends has been studied by means of differential scanning calorimetry (d.s.c.). It has been found that the following four states develop depending upon the heat-treatment temperature and the blend composition : (1) a miscible amorphous state in the melt-quenched blends, (2) a phase-separated amorphous state in the blend that had been heat treated at 260 degrees C, (3) a semicrystalline state with phase-separated amorphous regions in the blends that had been heat treated at 290-340 degrees C, and (4) a semicrystalline state with miscible amorphous regions in the blends that had been heat treated at temperatures above 350 degrees C. The semicrystalline morphology of the blends corresponding to the above four states was studied by means of small-angle X-ray scattering (SAXS). The results were analysed on the basis of the one-dimensional correlation function proposed by Stroble and coworkers. It has been found that the long-period and the amorphous thickness of the blends increase with increasing PEI content. The d.s.c. and SAXS studies of the blends lead to the conclusion that the PEI segments are incorporated in the amorphous layer between the stacked N-TPI crystals, regardless of the phase separation.