Nonregenerable reducing agents used in dye houses for the application of indigo, vat dyes and sulfur dyes can be substituted by indirect cathodic reduction techniques. An electrochemical cell for the indirect cathodic reduction of dispersed indigo dyestuff using an alkaline solution of the Fe(III/II)-triethanolamine complex as redox mediator was constructed and tested. The cell is built up as divided cell (cathode area 5-10 m(2), catholyte volume 12 L, anolyte volume 1.5 L) with several three dimensional cathode units (up to 10) in the same cathode compartment. The cathodes were connected to a common anode and to separately adjustable power supplies. The catholyte was circulated through the porous cathode units parallel to the current direction. Two different electrode materials (copper and stainless steel) and cathode constructions were tested, resulting in an optimized cell construction. The electrochemical cell was characterized by a series of batch electrolysis experiments. Results are given dealing with the cell voltage drop in the cathode, the product yield and the current efficiency at different current densities and cell current. After an optimization step the current efficiency reaches 70-80% at 2 A m(-2) current density and 7.8 x 10(-3) moldm(-3) Fe(II)-complex. The cell current is 10A.