Desorption is the reverse of adsorption, where adsorbed complexes dissociate, liberating one or more fluid-phase species and active sites. Modeled as a chemical reaction, the desorption of adsorbed molecule A would be represented as follows:
(Note that this is the reverse of the first adsorption reaction). The same rules of kinetics and thermodynamics apply to adsorption events, just as they do to other chemical reactions in general.
Desorption can also involve multiple adsorbed species-site complexes, such as
(which is the reverse of dissociative adsorption).
Note that in both of these cases, molecular species are returned to the gas phase and previously bound-up active sites are now open to participating in more chemical events.