The crystalline morphology and structural development of aromatic polyimides during an optimum continuous thermal imidization procedure were examined by means of polarized optical microscopy and X-ray diffraction. During thermal imidization, 3,3,4,4'-benzophenonetetracarboxylic dianhydride/1,3-diaminobenzene polyimide samples formed complicated spherulites, which, in addition to zigzag Maltese crosses, also showed concentric extinction rings, which are characteristic of banded spherulites. The factors affecting the formation of banded spherulites were studied. The initial imidization conditions dramatically affected the formation of the banded spherulite morphology: slow heating (0.5 degrees C/min) or fast heating (20 degrees C/min) led to relatively small polyimide spherulites and less identifiable extinction rings. The morphological features were also affected by the molecular weight of the polyimide: higher molecular weight samples showed typical banded spherulites, whereas low-molecular-weight samples formed degenerated banded spherulites. In all the spherulites formed in 3,T,4,4'-benzophenonetetracarboxylic dianhydride/1,3-diaminobenzene polyimides, special zigzag Maltese crosses, instead of normal Maltese crosses, were observed. The relationship between the imidization procedure and the spherulite morphology formation was also studied. X-ray and Fourier transform infrared together revealed that after several minutes of thermal treatment, the crystallization was nearly complete, with a 42.5% degree of crystallinity; meanwhile, only some poly(amic acid) converted to the corresponding polyimide, with a 27% degree of imidization. The crystalline morphology and structure formed in the initial stage of the imidization process were maintained during the following imidization processing at an elevated temperature. (c) 2005 Wiley Periodicals, Inc.