Cationic ring-opening polymerization of 3-ethyl-3-hydroxymethyloxetane (EOX) in a neutral ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4]) leading to a multihydroxyl, branched polyether proceeds readily to nearly quantitative conversion. Because of the relatively high polarity of ionic liquids, intermolecular hydrogen bonding leading to the formation of aggregates is reduced considerably. On the other hand, intramolecular hydrogen bonding facilitating intramolecular chain transfer is not significantly affected and the molecular weights of polymers are in the same range as those obtained in bulk polymerization or polymerization in organic solvents.