organic nitrogenous compounds of the general formula (where Ar is an aromatic or heterocyclic radical; X is the residue of a strong acid such as HSO₄^-, Cl^-, or BF₄^-). Diazonium salts are usually produced by the diazotization reaction: NaNO₂ acts on aromatic amines in an excess of strong acid, for example,

Almost all diazonium salts are solids; they are colorless or yellowish and readily soluble in water. They are highly reactive and therefore cannot be stored for long periods in aqueous solutions or as solids. In an acidic medium, diazonium salts break down (slowly in the cold and rapidly upon heating) into oxy compounds, nitrogen, and acid:

In solid form diazonium salts are explosive; they are usually processed in solution or suspension without isolation. Stable diazonium boron fluorides, , which are produced by the interaction of an alcohol solution of aromatic amine and fluoboric acid with nitrous acid esters (such as amyl nitrite), are an exception. Stable dry diazonium salts can be produced in the form of double salts (diazoles).

Nitrogen coupling, which is used to produce azo dyes, is the most important reaction of diazonium salts. Other reactions of diazonium salts, such as replacement of the diazo group with hydrogen, halogens, and other substances, are used in technology.