Tetrahydrofuran (THF) was polymerized with cocatalysts of ethylene oxide (EO) and BF3, using THF as the solvent. The yield of polymer, based on the amount of catalyst, was very low. With water present (0.5 mole per mole of BF3 and EO) the yield increased fourfold. Polymerization was stopped by the addition of a large amount of water; the addition of water also provided the polymer with terminal hydroxyl groups. The polymer had a molecular weight of 1000 and was a clear, colorless, viscous liquid.