The indirect cathodic reduction of dispersed vat dyes CI Vat Yellow 1 and CI Vat Blue 5 was investigated by cyclic voltammetry and spectroelectrochemistry. N,N-bis(2-hydroxyethyl)-3-amino-2-hydroxy-propane-sulfonic acid and 2,2-bis-(hydroxyethyl)-(iminotris)(hydroxymethyl) -methane were used as ligands to form electrochemically active Fe-complexes in alkaline solution. At a concentration of c(Fe3+) = 5 x 10(-4) mol dm(-)3 and ligand concentrations of 5 x 10(-3) mol dm(-3) in 0.1 mol dm(-3) NaOH cathodic peak potential (E-p)(d) of the complexes was observed between -1000 mV and -1020 mV. Reversible behaviour was found at a HMDE electrode. Suitability of the complexes for indirect cathodic dyestuff reduction was described with the so-called enhancement factors, ef, which are defined as the quotient of the cathodic peak current (I-p)(c) in the presence of dispersed dye and the diffusion controlled cathodic peak current of the mediator system (I-p)(d), thus ef = (I-p)(c)/(I-p)(d). Spectroelectrochemical experiments were used to prove indirect cathodic dyestuff reduction in case of very low enhancement factors of <= 1. (c) 2006 Elsevier B.V. All rights reserved.