Reaction behavior of ethylene oxide in the polymerization of tetrahydrofuran initiated by H3PW12O40 was studied. The polymerization started upon addition of ethylene oxide

and stopped with complete consumption of ethylene oxide

but started again when a new portion of EO was added

and also stopped after the new portion of EO was exhausted again. Part of ethylene oxide was transformed into dioxane at the fairly beginning of polymerization in the presence or absence of water

but the formation of dioxane could be suppressed by the addition of ethylene glycol. Kinetic data showed that the concentration of propagating species maintained constant during the polymerization with stepwise addition of EO. A reaction mwchanism was postulated based on the structure analysis of the product and kinetics study.