Process for preparing disodium 5,5′；-indigosulfonate, the feedstock being indigo, this process including the following steps: i) the indigo is subjected to a sulfonation treatment resulting in a mixture that contains 5,5′；-indigosulfonic acid, this process including: ii) a reduction treatment is applied to the mixture obtained in step i), and optionally a purification step, so as to obtain a composition including leuco-5,5′；-indigosulfonic acid, iii) the leuco-5,5′；-indigosulfonic acid is isolated from the composition resulting from step ii), iv) the leuco-5,5′；-indigosulfonic acid resulting from step iii) is oxidized to give disodium 5,5′；-indigosulfonate.
