Abstract: Use of metal complex dyes comprised of metalizable azo compounds to dye metal-tanned leather which makes possible good permeability and excellent color fastness to rubbing and color fastness to light.

Abstract: Metal complex dyes comprising a metalizable azo compound represented by one of Formulas [I] through [IV]: ##STR1## In Formulas [I] through [IV], Ps represents a phosphono group in the form of a free acid or a salt; Sf represents a sulfo group in the form of a free acid or a salt; m is 0, 1 or 2. R.sup.1 represents a hydrogen atom or an amino group; X represents a hydroxyl group, an alkoxy group having a carbon number of 1 or 2 or a carboxyl group. Y represents a hydroxyl group, an alkoxy group having a carbon number of 1 or 2, a carboxyl group or a methyl group; R.sup.2 represents a hydrogen atom or a hydroxyl group; A represents a phenyl group or naphthyl group selected from the group comprising the following 1 through 4: ##STR2## In the above formulas 1 through 4, Sf has the same definition as above; n is 0, 1 or 2. R.sup.3 represents a hydroxyl group or a carboxyl group; R.sup.4 represents a hydroxyl group, a carboxyl group or an amino group; R.sup.5 and R.sup.