The invention relates to a method for electro-catalytical hydrogenation of vat dyes and sulphide dyes in aqueous solutions, and to an apparatus for carrying out said method. The inventive method is suitable for batch operation and continuous operation. It works entirely without any reducing agents and provides a substantially salt-free dye concentrations of up to 200 g/l. The dyed threads are characterized by good friction qualities and by a high weaving yield. As far as waste water is concerned, no toxic load occurs and the salt loads are much lower, thereby enabling the waste water to be recycled in a substantially less complex manner in comparison with other conventional dye systems. The invention also relates to an apparatus for carrying out said method.