A polyimide (PI) based on benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride, toluene diisocyanate (TDI), and 4,4′-methylenebis (phenyl isocyanate) (MDI) has been synthesized via a one-step polycondensation procedure. The resulting PI possessed excellent thermal stability with the glass transition temperature (Tg) 316°C, the 5% weight loss temperature (T5%) in air and nitrogen 440.4°C and 448.0°C, respectively. The pyrolysis behaviors were investigated with dynamic thermogravimetric analysis (TGA), TGA coupled with Fourier transform infrared spectrometry (TGA–FTIR) and TGA coupled with mass spectrometry (TGA–MS) under air atmosphere. The results of TGA–FTIR and TGA–MS indicated that the main decomposition products were carbon dioxide (CO2), carbonic oxide (CO), water (H2O), ammonia (NH3), nitric oxide (NO), hydrogen cyanide (HCN), benzene (C6H6), and compounds containing NH2, CN, NCO or phenyl groups. The activation energy (Ea) of the solid-state process was estimated using Ozawa–Flynn–Wall (OFW) method which resulted to be 143.8 and 87.8 kJ/mol for the first and second stage. The pre-exponential factor (A) and empirical order of decomposition (n) were determined by Friedman method. The activation energies of different mechanism models were calculated from Coats–Redfern method. Compared with the activation energy values obtained from the OFW method, the actual reaction followed a random nucleation mechanism with the integral form g(α) = −ln(1 − α). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40163.