An in depth study is performed on the origin of and influences on the formation of tetrahydrofuran (THF) during the first stage of the terephthalic acid (TPA) based synthesis of poly(butylene terephthalate) (PBT). Although many improvements on the synthesis process of PBT have been reported in literature to suppress this undesired side reaction, only few studies reported on the actual mechanism of the THF formation, which is not completely understood. Low molecular weight compounds have been used to model the side reactions occurring during the polymerization reaction. It could be concluded that, in contrast to previous reportage, only the THF formation from the monomer, 1,4-butanediol, is directly influenced by the use of TPA as a starting material for the production of PBT. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2435-2444, 2009