Thermal degradation of fluorine-containing Polyformal II from bisphenol AF and dichloromethane was studied by TG-DTA/FTIR, Pyr-GC/MS and Pyr-GC and compared with that of Polyformal I without fluorine atoms, which was derived from bisphenol A and dichloromethane. The thermal degradation of Polyformal II occurs in two stages, whereas that of Polyformal I occurs in a single stage. The major products of thermal decomposition of Polyformal I are alpha-methylstyrene, phenol, 4-hydroxy-alpha-methylstyrene, isopropyl phenol, and 4-isopropenyl anisole, whereas those of Polyformal II are pentafluoroisopropenyl benzene and pentafluoroisopropenyl anisole. Tne two-step decrease in weight of Polyformal II by thermal degradation is not caused by the different reactions, but results from the different vapor pressures, that is, the different rates of vaporization between the decomposition products, more volatile pentafluoroisopropenyl benzene and less volatile pentafluoroisopropenyl anisole.