Isotope labeling is a technique used by chemists of various fields to determine specific steps in reaction mechanisms. Essentially, one key element of the reaction is replaced within the same molecule by its corresponding isotope, and the reaction is allowed to progress as normal. However, as the isotope emits radioactive energy, in one form or another, it is detectable by various analytical instruments. Thus, it is possible to follow the reactive element as it moves from one molecule to another. One must be careful not to observe kinetic behaviors, however, since the rates of reactions can be altered by altering the isotopic mass of a reaction component.

A common example of isotope labeling is the reaction of phenol (C₆H₅OH, MM = 94 g) with water, shown below. By mixing phenol and water, nothing in particular is observed. However, replacing water with deuterated water (D₂O, where D = ²H) yields another reaction. Both reactions are shown below:

Figure 1. Isotope Labeling Reaction.
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