The thermal decomposition of polycarbonate (PC), PC containing resorcinol bis(diphenyl phosphate) (RDP), and PC-acrylonitrile-butadiene-styrene (PC-ABS) blend containing RDP was studied by thermogravimetry. Volatile and solid products of thermal decomposition were collected at different steps of thermal decomposition and characterized either by gas chromatography-mass spectrometry or infrared and chemical analysis. It was found that phosphorus accumulates in the condensed phase. Upon combustion of the fire-retardant mixture PC-ABS + RDP, accumulation of phosphorus is observed in the charred layer, at the surface of the burning specimens. It is suggested that PC undergoes a Fries-type rearrangement upon thermal decomposition, and RDP reacts with the formed phenolic groups through a trans-esterification mechanism. Kinetic analysis of the thermal decomposition of PC containing RDP supports the proposed mechanism.