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The azo functional group is composed of two double bonded nitrogen atoms: R-N=N-R'. The R and R' groups of azo compounds are often aromatic. This helps to stabilise the N=N group by making it part of an extended delocalised system. This also has the effect of making many azo compounds coloured, as delocalised or conjugated systems often absorb visible frequencies of light.

Azo compounds are formed as the result of diazonium coupling reactions between diazonium salts and coupling agents in alkaline solution. The diazonium ion acts as an electrophile and reacts with the benzene ring of the coupling agent. Diazonium salts decompose above about 5 degrees centigrade so the reaction takes place in solution under freezing conditions: