The reaction behavior of water and low molecular weight glycol in tetrahydrofuran polymerization initiated by H3PW12O40 in the presence of ethylene oxide has been studied. A lot of water was used in the hydrolysis reaction of ethylene oxide at the early stage of the polymerization and transformed into ethylene glycol (EG), which was consumed subsequently through a chain transfer reaction. EG was more reactive both than 1,4-butylene glycol and hexamethylene glycol toward propagating species, and the reaction rate constants at 0 degrees C were determined by GC to be 0.142, 8.83 x 10(-2), and 5.53 x 10(-2) L mol(-1).s(-1), respectively. The molecular weight of the product can be predicted by an equation based upon conversion of polymerization and the concentrations of molecular weight controller and H3PW12O40.