Indigo is dissolved in water in the concentration of 5 x 10(-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/lencoindigo redox couple dissolved in the aqueous 0.7 M KNO3, pH 12 (NaOH) is - 0.565 +/- 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.