p-Phenylenediamine (PPDA) homopolymer and its copolymers with 2,3-xylidine (XY) were synthesized by oxidative polymerization using potassium persulfate as an oxidant in HCl medium at room temperature. The yield, intrinsic viscosity, and solubility of the polymers were significantly dependent on the monomer ratio. The resulting polymers were characterized by Fourier transform IR spectroscopy, H-1-NMR spectroscopy, wide-angle X-ray diffraction, and thermogravimetry methods. The results showed that the number-average degree of polymerization of the PPDA homopolymer was 33 and the actual content of XY units in the copolymer was slightly higher than the feed XY unit content. The polymers were substantially amorphous and showed the strongest diffraction at a Bragg angle of 3 degrees. The polymers exhibited a thermal decomposition temperature higher than 436 degreesC, the maximum weight-loss rate was slower than 4%/min, and the char yield was larger than 24 wt % at 600 degreesC in nitrogen. The activation energy of thermal decomposition for the polymers increased from 19 to 25 kJ/mol with increasing XY unit contents from 0 to 10 mol %. The polymers showed higher thermostability but lower activation energy of decomposition in nitrogen than in air.