A variation of the Sandmeyer reaction for preparing chloro- or bromoarenes by reaction of the diazonium compound. It was discovered in 1890 by the German chemist Ludwig Gattermann (1860-1920).

In the Gattermann reaction the aromatic amine is added to sodium nitrite and the halogen acid (10°C), then fresh copper powder is added and the solution warmed. The diazonium salt forms the haloarene. The copper acts as a catalyst.

The reaction is easier to perform than the Sandmeyer reaction and takes place at lower temperature, but generally gives lower yields.