Benzidine may be combined with all the phenols and amines which are used for the production of azo colours. It has been found that only one of the two azo groups of tetrazo-benzidine reacts vigorously, the other being relatively inert. In this way it is possible to prepare not only benzidine colours which have a single phenol or amino component, but also in many cases the so-called "mixed" benzidine dyes. Such products, however, are only formed if the first dye component does not react too readily, as otherwise some of the disazo dye will be formed in addition to the intermediate compound:

It would take far too long to mention even the most important types of such intermediate products, so that we must therefore confine ourselves to a few typical examples. Of particular importance is the intermediate compound which is formed by the combination of a single equivalent of salicylic acid with benzidine. This is formed very simply, as a second salicylic group can only be introduced with difficulty after the sodium carbonate coupling by means of an excess of caustic soda solution. Further, it is possible to couple benzidine once with the monoazo dye p-nitraniline→H-acid, or with H-acid alone in mineral acid solution without difficulty. Both cases are discussed in detail below.