The concept of selectivity is used to quantify the extent to which one chemical substance, A, binds each of two other chemical substances, B and C. The simplest case is where the complexes formed have 1:1 stoichiometry. Then, the two interactions may be characterized by equilibrium constants  and .
where [X] represents the concentration of substance X (A, B, C, …).
A selectivity coefficient is defined as the ratio of the two equilibrium constants.
This selectivity coefficient is in fact the equilibrium constant for the displacement reaction
It is easy to show that the same definition applies to complexes of a different stoichiometry, AB and AC. The greater the selectivity coefficient, the more the ligand C will displace the ligand B from the complex formed with the substrate A. An alternative interpretation is that the greater the selectivity coefficient, the lower the concentration of C that is needed to displace B from AB. Selectivity coefficients are determined experimentally by measuring the two equilibrium constants,  and .