The invention relates to novel diaminopyrazole compounds having formula (I): wherein R.sub.1 represents a linear or branched C.sub.4-C.sub.5 alkyl or n-propyl group, or a linear or branched C.sub.3-C.sub.5 mono or polyhydroxy alkyl group, and the addition salts thereof with a physiologically acceptable acid. The invention also relates to the compositions containing such a compound for the dyeing of keratinous fibres and the method for using said compositions.