Thermoplastic poly(imide‐siloxanes) (PISILs) were synthesized from hexafluoroisopro‐pylidene‐2,2‐bis(phthalic acid anhydride) (6FDA), 3,3',4,4'‐benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), and amine‐terminated poly(dimethyl siloxanes) (PDMS) having different molecular weights. Glass transition temperatures of fully cured PISILs ranged from −91 to −48°C. Various kinetic parameters for the thermal decomposition reaction of PISILs were examined by dynamic thermogravimetric analysis (DTGA) at four different heating rates. The degradation path was a kinetically first‐order reaction for PISILs (abbreviated 6FDA‐860, BTDA‐860, and PMDA‐860, respectively) from 6FDA, BTDA, PMDA, and PDMS (Mn = 860 g/mol). The reaction rate constant times the half‐life (k × t1/2) was approximately 0.693. The activation energies for decomposition were in the following order: 6FDA‐860 <BTDA‐860 <PMDA‐860. The lifetime of the highest heat‐resistant PMDA‐860 at about 200°C and at the conversion level of 10% was 6.5 × 105 h or 74 years. Other kinetic factors were also calculated. © 1993 John Wiley & Sons, Inc.