Several isotopes of both hydrogen and oxygen exist, giving rise to several known isotopologues of water. Vienna Standard Mean Ocean Water is the current international standard for water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Only 155 ppm include deuterium ( or D), a hydrogen isotope with one neutron, and fewer than 20 parts per quintillion include tritium ( or T), which has two neutrons. Oxygen also has three stable isotopes, with  present in 99.76%,  in 0.04%, and  in 0.2% of water molecules.
Deuterium oxide, , is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. Tritium is radioactive, decaying with a half-life of 4500 days;  exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. Water with one protium and one deuterium atom  occur naturally in ordinary water in low concentrations (~0.03%) and  in far lower amounts (0.000003%) and any such molecules are temporary as the atoms recombine.
The most notable physical differences between  and , other than the simple difference in specific mass, involve properties that are affected by hydrogen bonding, such as freezing and boiling, and other kinetic effects. This is because the nucleus of deuterium is twice as heavy as protium, and this causes noticeable differences in bonding energies. The difference in boiling points allows the isotopologues to be separated. The self-diffusion coefficient of  at 25 °C is 23% higher than the value of . Because water molecules exchange hydrogen atoms with one another, hydrogen deuterium oxide (DOH) is much more common in low-purity heavy water than pure dideuterium monoxide .
Consumption of pure isolated  may affect biochemical processes—ingestion of large amounts impairs kidney and central nervous system function. Small quantities can be consumed without any ill-effects; humans are generally unaware of taste differences, but sometimes report a burning sensation or sweet flavor. Very large amounts of heavy water must be consumed for any toxicity to become apparent. Rats, however, are able to avoid heavy water by smell, and it is toxic to many animals.
Light water refers to deuterium-depleted water (DDW), water in which the deuterium content has been reduced below the standard  level.