The flame retarding mechanism for polycarbonate (PC) by a trifunctional phenyl-rich silicone additive was studied by analytical pyrolysis techniques. In order to clarify the actions of the silicone-based flame retardant for PC substrate during combustion, the change in the chemical structure of the flame retarded PC with the silicone additive (FR-PC) after thermal treatment at 380 degreesC was investigated by pyrolysis-gas chromatography (Py-GC) in the presence of organic alkali. On the pyrogram of the thermally treated FR-PC, which exhibited highly insoluble nature, the peaks reflecting the abnormal structures, formed through the reaction between a silyl radical originating from the additive and an ether like oxygen atom in the carbonate linkage of the PC chain accompanied by decarboxylation or Fries rearrangement, were clearly observed in much larger intensity than those on the pyrogram, of the thermally treated PC. On the basis of the observed results for the thermally treated FR-PC, it was suggested that the formation of a char barrier on the surface of the FR-PC material was promoted during combustion to reduce the radiant heat of flame and to restrict the diffusion of flammable degradation products into the combustion zone. Thus formed cross-linking structure might surpress the thermal decomposition and confine the movements of the degradation products.