The binding constant is a special case of the equilibrium constant K. The equilibrium state of molecular binding, i.e. the balance between the binding and dissociation processes after infinite reaction time, may be formalized as the unbound compounds (reactants, A and B) transforming into a complex (product, C):

A + B \rightleftharpoons C

This directionality of reaction defines the association equilibrium constant K_a:

K_{ass} = {k_{on} \over k_{off}} = {[C] \over [A] \times [B]} (second order or bimolecular association)

Since the binding constant is related to the molar free enthalpy, it is used to quantify the affinity of binding. For this purpose, the reverse directionality is used because the resulting formalism is a reaction of first order and thus its dimension is always in mol (or mol/L). This defines the dissociation equilibrium constant K_diss:

K_{diss} = {k_{off} \over k_{on}} = {[A] \times [B] \over [C]} (first order dissociation)