Direct cathodic reduction of azo dyes permits decolourization of intensively coloured textile wastewater without addition of chemicals or formation of sludge. The technique is of particular interest for the treatment of concentrated dyebaths from pad batch-dyeing with reactive dyes which contain on average about 50 g L−1 of dyestuff. On the basis of laboratory scale experiments with model dyebaths and dyebath samples from technical application a scale-up was performed. A multicathode electrolyser for a maximum cell current of 150 A successfully was tested in a dyehouse. The treatment capacity of the installed electrolyser was 1 m3 of used concentrated dyebath per day. This corresponds to the full amount of such concentrates which have to be treated in a medium sized dyehouse. By cathodic decolourization the absorbance of the concentrated dyebaths was lowered to approximately 20% of the initial value with an energy consumption of 20–25 kWh m−3.