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.