Abstract: Process for preserving the dyeing capacity of 5,6-dihydroxyindole or of its derivatives corresponding to the formula (I): ##STR1## in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which are identical or different, denote hydrogen or a C.sub.1 -C.sub.4 lower alkyl, R.sub.1, R.sub.2 and R.sub.3 denoting hydrogen when R.sub.4 denotes alkyl in an aqueous medium, characterized in that the pH of the aqueous medium is adjusted to a value pH.sub.C of between 5 and 10 by employing a pH-regulating agent containing two components (A) and (B), the component (A) being chosen from pH-regulating agents which have a dissociation constant K.sub.CA in aqueous solution at 25.degree. C., such as pk.sub.CA, that is:0<pk.sub.CA -pH.sub.C .ltoreq.2.5and the component (B) being chosen from pH-regulating agents which have a dissociation constant K.sub.CB in aqueous solution at 25.degree. C., such as pk.sub.CB, that is:2<pk.sub.CB <pH.sub.C.