The phase behaviour of poly(ester imide)s (PEI) based on long aliphatic diols and aminobenzoic acid trimellitimide or aminocinnamic acid trimellitimide, respectively, was investigated by means of X-ray scattering. Whereas PEI with shorter spacers form smectic LC-phases which transform into higher ordered smectic phases upon further cooling, PEI with long spacers undergo a direct phase transition from the isotropic melt into a higher ordered solid smectic phase, which is also named smectic-crystalline owing to the lateral interactions of the mesogens. This process involves a nucleation followed by the growth of a three-dimensional spherulitic superstructure. This new morphology of spherulites with internal smectic order is named smectic-spherulitic. Moreover, the spherulites consist of stacks of crystalline and lower-ordered lamellae giving rise to a small angle X-ray reflection with a long period of 150-400 Angstrom. The crystalline lamellae in their part consist of smectic layers. In spite of the crystallinity, the system exhibits a remarkable mobility. Upon heating and cooling the lamellae expand and shrink reversibly. This polymer system combines features of semi-crystalline polymers with the mesophase character of smectic LC-polymers. (C) 1997 Elsevier Science Ltd.