The hydrolytic behaviors of polyphenyl-1,2,4-triazine (As-PPT) at high temperatures were investigated both experimentally and theoretically. The hydrolytic experiments of As-PPT film and a reasonably designed model compound for the polymer were carried out in distilled water at 250 degreesC/3.97 MPa under N-2 atmosphere, respectively. The hydrolytic reactions were monitored by FTIR and UV-Vis spectra, and their hydrolysates were identified by FTIR, HPLC, and MS. The results confirmed that the two reactions gave the same major hydrolysate, terephthalic acid. In addition, the electronic structure of its model compound, given by ab initio calculation at the 4-31G level, indicated that the triazine ring is the hydrolytically active part at which the N atoms are to be protonated. On the basis of these results, a hydrolytic mechanism is proposed, which suggests that the cleavage of one C-N bond takes place, primarily on the triazine rings, followed by a gradual rupture of the polymer chain and the formation of the intermediate terephthalonitrile, the further hydrolysis of which produces terephthalic acid.