Heavy_water Heavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O.Heavy water may contain as much as 100% D2O, and usually the term refers to water that is highly enriched in deuterium.