An example of the use of isotopic labeling is the study of phenol (C 6 H 5 OH) in water by replacing common hydrogen (protium) with deuterium (deuterium labeling). Upon adding phenol to deuterated water (water containing D 2 O in addition to the usual H 2 O), the substitution of deuterium for the hydrogen is observed in phenol's hydroxyl group (resulting in C 6 H 5 OD), indicating that phenol readily undergoes hydrogen-exchange reactions with water. Only the hydroxyl group is affected, indicating that the other 5 hydrogen atoms do not participate in the exchange reactions.