Quantum chemical methods have been used to identify reaction pathways of the thermal decomposition of bisphenol C polycarbonate, one of the most fire-resistant polymers known to the scientific community. Despite substantial interest in its unusual high-temperature behavior, the mechanism of its thermal decomposition has been unknown. On the basis of computational results, a mechanism is proposed where the main feature is a shift of Cl atom from the P-styrene position to the adjacent aromatic ring, which leads to crosslinking and cyclization of the polymer. The proposed mechanism is consistent with experimental observations of char, HCl, and CO2 as the main pyrolysis products. (C) 2003 Elsevier Ltd. All rights reserved.