The reactive pyrolysis-gas chromatography technique was applied to verify the branching and/or cross-linking structures in an industrially available PC sample and its thermally treated ones through identification of specific pyrolysis products directly reflecting the related abnormal structures. On the pyrograms of the thermally treated samples, the peaks reflecting the abnormal structures such as branching and/or cross-linking were observed together with those reflecting main chain and end groups. Although the formations of these branching and/or cross-linking structures during the thermal treatment of PC had been suggested early, the identification of these pyrolysis compounds by reactive pyrolysis verified the existence of these structures. Furthermore, the fact that some of those characteristic peaks were also observed on the pyrogram of the industrially available PC sample prepared by the melt method indicated that the branching and/or cross-linking reactions would occur to some extent in the industrial polymerization reactor to synthesize the PC by the melt method.