The invention concerns compounds derived from diaminopyrazole of formula (I), wherein: Ar is a 5- or 6-membered heteroaromatic radical capable of being condensed with one or several heteroaromatic cycles each comprising 5 or 6 members, and their physiologically acceptable addition salts with and acid. The invention also concerns compositions containing such a compound for oxidation dyeing of keratinous fibers and the method using said compositions ##STR00001##