A series of polyamides and polyimides containing pendent benzoxazole or benzothiazole pendent groups were prepared from two new monomers, 5-(2-benzoxazole)- or 5-(2-benzothiazole)-1,3-phenylenediamine. In addition, model compounds were synthesized and characterized by elemental analyses, FT-IR, and H-1-NMR. Characterization of polymers was accomplished by inherent viscosity measurements, FT-IR, X-ray, DTA, TGA, and isothermal gravimetric analysis. Water absorption studies were also carried out for the polyamides. The properties of the modified polymers were correlated with those of the corresponding unmodified ones. The solubilities of modified polyamides in common organic solvents as well as their hydrophilicities were enhanced compared to these of the corresponding unmodified polyamides. The synthesized polyamides softened between 200 and 300 degrees C, but no softening was observed for the polyimides. The modified polyamides and polyimides suffered less weight loss under TGA and isothermal gravimetric analysis than the parent structures. Their initial decomposition temperatures in N-2 or air and the anaerobic char yields at 800 degrees C were, respectively, 24-68 degrees C and 5-9% higher than those of the unmodified polymers.