A series of terpolymers were synthesized by oxidative polymerization of aniline, o-toluidine, and 2,3-xylidine with three monomer ratios using ammonium persulfate as an oxidant in two acidic media. The yield and intrinsic viscosity of the terpolymers were studied by changing the polymerization temperature and medium. The resulting terpolymers were characterized by H-1-NMR, wide-angle X-ray diffraction, differential scanning calorimetry, thermogravimetry, and constant pressure-variable volume methods. The results showed that the terpolymers are amorphous and exhibit enhanced solubility, high thermostability, and high char yield, greater than 35 wt % at 900 degreesC in nitrogen. A blend membrane of the terpolymer with ethyl cellulose shows good air-separation ability.