If a reaction occurs through these steps:
: A + S ⇌ AS  → Products
where A is the reactant and S is an adsorption site on the surface and the respective rate constants for the adsorption, desorption and reaction are k, k and k, then the global reaction rate is:
where:
* r is the rate, mol·m·s
*is the concentration of adsorbate, mol·m
* is the surface concentration of occupied sites, mol·m
* is the concentration of all sites (occupied or not), mol·m
* is the surface coverage, (i.e. ) defined as the fraction of sites which are occupied, which is dimensionless
* is time, s
* is the rate constant for the surface reaction, s.
*is the rate constant for surface adsorption, m·mol·s
*is the rate constant for surface desorption, s
 is highly related to the total surface area of the adsorbent: the greater the surface area, the more sites and the faster the reaction. This is the reason why heterogeneous catalysts are usually chosen to have great surface areas (in the order of a hundred m/gram)
If we apply the steady state approximation to AS, then:
:  so 
and
The result is equivalent to the Michaelis–Menten kinetics of reactions catalyzed at a site on an enzyme. The rate equation is complex, and the reaction order is not clear. In experimental work, usually two extreme cases are looked for in order to prove the mechanism. In them, the rate-determining step can be:
*Limiting step: adsorption/desorption
The order respect to A is 1. Examples of this mechanism are NO on gold and HI on platinum
*Limiting step: reaction of adsorbed species
The last expression is the Langmuir isotherm for the surface coverage. The adsorption equilibrium constant , and the numerator and denominator have each been divided by . The overall reaction rate becomes .
Depending on the concentration of the reactant the rate changes:
:* Low concentrations, then , that is to say a first order reaction in component A.
:* High concentration, then . It is a zeroth order reaction in component A.