The thermooxidative aging of ammonia-catalyzed phenolic resin for 30 days at 60170 degrees C was investigated in this article. The aging mechanism and thermal properties of the phenolic resin during thermooxidative aging were described by thermogravimetry (TG)Fourier transform infrared (FTIR) spectroscopy, attenuated total reflectance (ATR)FTIR spectroscopy, and dynamic mechanical thermal analysis. The results show that the C?N bond decomposed into ammonia and the dehydration condensation between the residual hydroxyl groups occurred during the thermooxidative aging. Because of the presence of oxygen, the m(e)thylene bridges were oxidized into carbonyl groups. After aging for 30 days, the mass loss ratio reached 4.50%. The results of weight change at high temperatures coincided with the results of TGFTIR spectroscopy and ATRFTIR spectroscopy. The glass-transition temperature (Tg) increased from 240 to 312 degrees C after thermooxidative aging for 30 days, which revealed the postcuring of phenolic resins. In addition, an empirical equation between the weight change ratio and Tg was obtained. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012