Also known as the two-site Langmuir equation. This equation describes the adsorption of one adsorbate to two or more distinct types of adsorption sites. Each binding site can be described with its own Langmuir expression, as long as the adsorption at each binding site type is independent from the rest.
where
:  – total amount adsorbed at a given adsorbate concentration,
:  – maximum capacity of site type 1,
:  – maximum capacity of site type 2,
:  – equilibrium (affinity) constant of site type 1,
:  – equilibrium (affinity) constant of site type 2,
:  – adsorbate activity in solution at equilibrium
This equation works well for adsorption of some drug molecules to activated carbon in which some adsorbate molecules interact with hydrogen bonding while others interact with a different part of the surface by hydrophobic interactions (hydrophobic effect). The equation was modified to account for the hydrophobic effect (also known as entropy-driven adsorption):
The hydrophobic effect is independent of concentration, since  Therefore, the capacity of the adsorbent for hydrophobic interactions  can obtained from fitting to experimental data. The entropy-driven adsorption originates from the restriction of translational motion of bulk water molecules by the adsorbate, which is alleviated upon adsorption.