Kinetic fractionation is a process that separates stable isotopes from each other by their mass during unidirectional processes.

One naturally-occurring example of kinetic fractionation is the evaporation of seawater to form clouds. In this instance, isotopically-lighter water molecules (i.e., those with ¹⁶O) will evaporate slightly more easily than will the isotopically-heavier water molecules with ¹⁸O.

During the course of this process the oxygen isotopes are fractionated: the clouds become enriched with ¹⁶O, the seawater becomes enriched in ¹⁸O. Thus, rainwater is observed to be isotopically lighter than seawater.

Unit operations | Environmental isotopes