Indigo is dissolved in water in the concentration of 5x10 - 5 M and measured by highly sensitive adsorptive stripping square-wave voltammetry in order to determine the parameters of its electrochemical reduction. The formal potential of the indigo/leucoindigo redox couple dissolved in the aqueous 0.7 M KNO 3 , pH 12 (NaOH) is -0.566+/-0.003 V versus Ag|AgCl|3 M KCl. Both indigo and leucoindigo are adsorbed on the surface of the mercury electrode, and the standard rate constant of this redox reaction is 920+/-130 s - 1 . The ratio of adsorption constants of indigo and leucoindigo is 0.75.