The invention concerns novel diaminopyrazole derivatives of formula (I), wherein: R.sub.1 represents C.sub.1-C.sub.6 alkyl or C.sub.2-C.sub.6, preferably up to C.sub.4, alkenyl, linear or branched, bearing at least a substituent selected among OR, NHR, NRR', SR, SO.sub.2R, COOH, CONH.sub.2, CONRR', PO(OH).sub.2, SH, SO.sub.3X, a non-cationic heterocycle, Cl, Br or I.sup.-; X represents H, Na, K or NH.sub.4; R and R', identical or different, represent C.sub.1-C.sub.6 alkyl or C.sub.2-C.sub.6, preferably up to C.sub.4, alkenyl, linear or branched, unsubstituted or substituted by one or several functional groups selected among OH, NH.sub.2, OR, NHR, NRR', SR, SOR, SO.sub.2R, COOH, CONH.sub.2, CONRR', PO(OH).sub.2, SH, SO.sub.3X, a non-cationic heterocycle, Cl, Br or I, X, R and R' having the definitions mentioned above; R.sub.2 represents H, or C.sub.1-C.sub.6 alkyl or C.sub.2-C.sub.6, preferably up to C.sub.4, alkenyl, linear or branched, unsubstituted or substituted by one or several functional groups selected among OH, NH.sub.2, OR, NHR, NRR', SR, SOR, SO.sub.2R, COOH, CONH.sub.2, CONRR', PO(OH).sub.2, SH, SO.sub.3X, a non-cationic heterocycle, Cl, Br or I, X, R and R' having the definitions mentioned above; and their physiologically acceptable salts, and their use for oxidation dyeing of keratinous fibres, in particular human hair.