Poly(butylene terephthalate) (PBT) was decomposed into terephthalic acid (TPA) and tetrahydrofuran (THE) in high-temperature liquid water above 498K. THE was obtained by the dehydration of the monomer, 1,4-butanediol (1,4-BDO). The yields of TPA, THF, and 1,4-BDO were 98%, 80%, and 12%, respectively, at 523 K after 1 h of the PBT depolymerization in water. The depolymerization rate was enhanced by the additional TPA, which was one of the products.