The presence of intermolecular donor-acceptor interaction in solid state aromatic polyimides (PIs) was confirmed by spectroscopic means. It was found that the thermal expansion coefficient is closely associated not only with the degree of in-plane orientation, but also with the degree of molecular stacking. These two factors influence each other, and are not completely independent. The intermolecular CT fluorescence spectra were a sensitive reflection of the molecular stacking process. Dynamic mechanical measurements of PIs suggested that the CT complexes probably function as physical crosslinks.Aromatic polyimides (Pis), especially those possessing a rigid (linear) polymer backbone, e.g., PI(BPDA/PDA) (BPDA: 3,4,3\4 f ,-biphenyltetracarboxylic dianhydride, PDA: /?-phenylenediamine), show properties depending strongly on the cure method (chemical and thermal), thermal imidization conditions (final cure temperature and heating rate), presence of substrate, and film thickness. This is attributed to the difference of the degree of molecular orientation and molecular stacking which are closely related with molecular mobility during thermal cure of polyamic acid (PAA). The aim of the work is to elucidate the presence and contribution of intermolecular charge-transfer (CT) interaction on various physical properties and to examine the relationship between physical properties and the structures of molecular aggregates. The intermolecular charge-transfer (CT) fluorescence is useful as an indicator of the degree of PI chain packing(I-i) and the miscibility of PI/PI binary blends. (4) Recently, Okabe et al. (5) reported that a PI derived from a polyamic acid ester (PAE) has a thermal expansion coefficient (TEC) higher than the PI made from the corresponding PAA. The present paper also describes the difference of molecular aggregation structure between Pis from PAA and PAE, characterized by the intrinsic intermolecular CT fluorescence(!,3) and the dichroic absorption spectra measured at a tilt angle for a rigid-rod dye dispersed in PI film. (6) EXPERIMENTAL SECTION Materials. PAAs were polymerized by adding equimolar amounts of dianhydride powder into a iV,N-dimethylacetamide (DMAc) solution of diamine at room