Carbonization of oxidized mesophase pitches originating from petroleum and coal tar was examined by thermogravimetry, FT-i.r. spectroscopy, and quantitative analysis of evolved gases. Polymerization and condensation of aromatic rings through loss of aromatic hydrogen are shown to be major chemical reactions in carbonization of both oxidized pitches. Characteristic of oxidized pitches in carbonization is the evolution of considerable amounts of CO, CO2 and H,O caused by decomposition of oxygen-containing groups introduced by oxidation. Thermal decomposition of the oxygen-containing groups probably initiates polymerization of pitch molecules before the major chemical reactions. The polymerization promotes the conversion of a fusible fibre into an infusible one. Of the oxygen-containing groups, carboxyl is decomposed at the lowest temperature. The oxidized petroleum pitch contains a larger number of carboxyl groups than the oxidized coal-tar pitch because the former has more aliphatic structures which can be converted into carboxyl groups by oxidation. The results strongly suggest that the petroleum mesophase pitch is made infusible more easily than the coal-tar mesophase pitch.