A series of terpolymers were synthesized by the chemical oxidative polymerization of m-phenylenediamine (MPD), o-anisidine (AS), and 2,3-xylidine (XY) in hydrochloride aqueous medium. The yield, intrinsic viscosity, and solubility of the terpolymers were studied by changing the MPD/AS/XY molar ratio from 100/0/0 to 53/39/8 to 0/100/0. It was discovered that the MPD/AS= terpolymers exhibit a higher polymerization yield and better solubility than MPD/AS and MPD/XY bipolymers having the same MPD molar content. The as-prepared MPD/AS/XY terpolymer bases were characterized by Fourier transform infrared, ultraviolet-visible, H-1 NMR, and high-resolution solid-state C-13 NMR spectroscopies; wide-angle X-ray diffraction; and thermogravimetry. The results suggested that the oxidative polymerization from MPD, AS, and XY is exothermic, and the resulting terpolymers are more easily soluble in some organic solvents than MPD homopolymer. The copolymer obtained was a real terpolymer containing MPD, AS, and XY units, and the actual MPD/AS/XY molar ratio calculated by solid-state C-13 NMR spectra of the polymers is very close to the feed ratio, although the AS content calculated on the basis of the H-1 NMR spectrum of the soluble part of the polymer is higher than the feed AS content. The terpolymers and MPD homopolymer exhibit a higher polymerization yield and much higher intrinsic viscosity and are more amorphous than the AS homopolymer. At a fixed MPD content of 70 mol %, the terpolymers exhibit an increased thermostability and activation energy of the major degradation in nitrogen and air with an increasing AS content.