Abstract: The invention concerns a process for the purification of acid dyehouse effluent from a dyeing process, which is contaminated with coloured substances and heavy metal ions. After removal of fibers, the effluent is subjected to ion exchange to remove surface active agents and is then subjected to electrolysis with an iron cathode. Ferrous and ferric hydroxide is formed and the ferrous and ferric hydroxide together with the colored substances and heavy metal ions are removed from the effluent liquid. The liquid which remains is recycled to the dyehouse.