Oxymercuration–demercuration is a two-step organic chemical reaction process that adds water (H2O) across the double bond of alkenes, in a regioselective manner, to form alcohols without the rearrangement typically seen in acid-catalyzed hydration. First, oxymercuration involves the addition of mercury acetate and water across the alkene's double bond, followed by demercuration, where sodium borohydride reduces the intermediate to produce an alcohol.