Basic Brown 1,[1] is a diazo dye with the idealized formula [(H2N)2C6H3N2]2C6H4.

It was one of the earliest azo dyes, being described in 1863 by German chemist Carl Alexander von Martius.

The synthesis is thought to start with double diazotization of 1,3-phenylenediamine: It is assumed that this bis(diazonium) ion subsequently attacks two equivalents of 1,3-phenylenediamine: In some cases, toluenediamines are used in addition to the phenylenediamine.

Furthermore, the resulting dye is thought to consist of oligomers with three or more diazo groups.

[2] Bismarck brown Y stains acid mucins to yellow color.