A type of azo compound formed on the fibre by first treating the fibre with a phenolic compound. The fibre is then immersed in a second solution containing a diazonuim salt that reacts with the phenilic compound to produce a coloured azo compound. Since the phenolic compound is dissolved in caustic solution, these dyes are mainly used for cellulose fiber, although other fibres can be dyed by modifying the process.