Acid-base interaction-mediated compatibilization between rigid-rod and matrix polymer components facilitated the formation and processing of solvent cast aromatic heterocyclic rigid-rod thermoplastic as well as thermoset molecular composites above the critical concentration (C-cr) of the rigid-rod polymer in solution, without phase separation. The blends were solvent cast by the mechanism of ionic interchange between a sulfonic acid-pendent poly(p-phenylenebenzobisimidazole) (SPBI), solubilized in alcohol as its triethylammonium salt and basic thermoplastics such as poly(vinylpyridine)s or secondary or tertiary amines with thermosettable phenylethynyl, nadimide and bisbenzoxazine functionalities. Morphological characterization, utilizing SEM, WAXS and SAXS of as cast as well as annealed/thermally cured optically clear film composites of a broad range of compositions revealed homogeneous microstructures with no observable phase-separated domains, indicating high miscibility, ascribable to the favorable negative enthalpy of the ionic association between the rod-matrix components. A preliminary dynamic mechanical study of compression molded rigid-rod thermoplastic blends with relatively low rod contents showed significant enhancement in thermomechanical properties vis-a-vis the pristine matrix. (C) 2000 Elsevier Science Ltd. All rights reserved.