<p>The following question is considered—why in dinitrosyl iron complexes functioning in living organisms dinitrosyl ligands are represented as the “working form” of endogenous nitric oxide (NO), moreover, in equal proportions of neutral NO molecules and their cationic (NO⁺) form? It is shown that this representation is a direct consequence of the mechanism of formation of dinitrosyl iron complexes occurring in living organisms with the participation of NO molecules, weakly bound ferrous iron and thiol-containing compounds. The fundamental stage of this process is the disproportionation reaction of NO molecules that bind in pairs to the Fe²⁺ ion. As for thiol-containing ligands, their presence in dinitrosyl iron complexes ensures the stabilization of nitrosonium cations that appear in these complexes during the reaction of disproportionation of NO molecules.</p>