Using D2O as a deuterium source, a method for the deuteration of intra- and extra-cyclic methylene has been developed for cyclic ethers with moderate yield and excellent deuterium incorporation. This transformation features superb functional group tolerance in a wide range of alkynols. Notably, the critical factor to achieve high deuterium incorporation is determined by the hydrogen isotope exchange reaction of an unstable oxonium ion. This novel methodology provides an efficient and concise synthetic route to a number of valuable deuterated cyclic ethers that are often difficult to prepare with other methods.