The peculiar buffering behaviour of the “background sites” of natural heterogeneous complexants is described. Background sites include most of the minor sites, i.e., a group of sites typically comprising not more than ca. 10% of the total sites. Despite this low proportion, they play an important role in controlling free metal ion concentrations in natural systems, because their complexation energy is higher than that of the major sites and their log K values span a broad range of order of magnitudes, owing to the cooperative effect of the large number of site types that they include, together with such continuous secondary effects as those due to a polyelectrolytic interactions and conformational changes. The main characteristics of these sites are summarized and a mathematical and chemical explanation for their cooperative action in controlling the overall free energy of metal ion complexation and the overall buffering intensity of the system is presented. Comparison is made with a classical system made up of a few simple ligands and devoid of secondary effects. The important parameters useful for describing the properties of background sites as a whole are deduced for complexation data obeying the Freundlich isotherm, which has been shown to fit most of the properties of background sites of natural systems.