Ordinary water consists of two hydrogen atoms and one oxygen atom. Heavy water consists of two deuterium atoms and one oxygen atom. Deuterium is an isotope of hydrogen; both have the same number of protons, and therefore have the same atomic number. However, deuterium has one neutron in its nucleus while hydrogen has none and so deuterium is heavier than hydrogen. Thus a molecule of heavy water has a mass of 20 atomic units while an ordinary water molecule has a mass of 18 units. Pure heavy water is 10% more dense than ordinary water; it freezes at 38.8°F (3.8°C) and boils at 214.5°F (101.4°C). It is not radioactive and occurs naturally in nature in limited quantities.

Heavy water, which can be harvested from natural water sources or created from electrolyzed water, is used as a moderator in nuclear power plants. The uranium-235 in the reactor can capture neutrons only if they are moving slowly enough; thus a moderator is needed to slow the particles. High-speed neutrons hit the deuterium and lose some of their momentum, slowing the neutron. Ironically, slowing down the neutrons will increase the nuclear fission rate. Ordinary water is used as a moderator as well, but absorbs most neutrons instead of just slowing them through collisions; these reactors must use enriched uranium to be able to sustain nuclear fission. Other uses for deuterium include quantum effect experiments, extensive use in molecular structure studies, superconductivity experiments, chemical kinetics, nuclear magnetic resonance shift studies, dipole moment studies, ESR hyperfine interactions, geochemical experiments, and experiments dealing with growth and morphology of plants and animals.