In this work, poly(amide-amidic acid) (PAA) was used to modify tetraglycidyl 4,4-diaminodiphenylmethane (TGDDM)/4,4-diaminodiphenylsulfone (DDS) system. Results of non-isothermal differential scanning calorimetry analysis indicated that PAA played a role of catalyst during the process of the curing reaction. The curing mechanism was studied by Fourier transform infrared spectroscopy, showing that the PAA acted as a co-curing agent in the system. The glass transition temperature decreased firstly and then increased with the increase of the PAA content. PAA equally rendered TGDDM more fire resistant with higher char yield. On examining the fracture surface morphology using scanning electron microscopy, it was observed that there was no obvious phase separation when the content of PAA was less than 20 phr (per hundred weight of TGDDM/DDS resin), however, phase separation was observed when the content of PAA was 25 and 30 phr. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013