Chemically oxidative polymerization of aniline and o-toluidine (OT) with 6 molar ratios was performed using ammonium persulfate as an oxidant in a HCl medium at -18 degreesC. It was observed that the copolymers were not regularly dependent on the monomer ratio for yield and intrinsic viscosity. The resulting copolymers were characterized by Fourier transform IR spectroscopy, H-1-NMR spectroscopy, wide-angle X-ray diffraction, differential scanning calorimetry, and several thermogravimetry methods. The results showed that the actual content of the OT unit in the copolymers was much higher than the feed content of the OT unit. Given the same reaction conditions, o-toluidine showed a larger tendency toward homopolymerization rather than toward copolymerization with aniline. The copolymers were amorphous and exhibited thermal decomposition temperatures higher than 440 degreesC and a char yield larger than 39 wt % at 900 degreesC in nitrogen. The activation energy of thermal decomposition for the copolymers was 40-244 kJ/mol in nitrogen and 32 kJ/mol in air, respectively.