The invention relates to novel diaminopyrazole having formula (I), wherein: R.sub.1, R.sub.2, identical or different, represent a C.sub.3 C.sub.5 mono- or polyhydroxyalkyl, isopropyl, n-propyl or ethyl group, said groups being linear or branched, and their physiologically acceptable salts. The invention also relates to a composition for dyeing keratinous fibres containing a compound having formula (I) and the method for using same. ##STR00001##