Azo dyes of the formula ##STR1## in which R.sup.1, R.sup.2 and R.sup.3 independently denote hydrogen, C.sub.1 -C.sub.4 -alkyl or C.sub.1 -C.sub.4 -alkoxy,R.sup.4 denotes C.sub.1 -C.sub.6 -alkyl andR.sup.5 denotes C.sub.2 -C.sub.8 -alkyl which is substituted by hydroxy and may be interrupted by 1, 2 or 3 oxygen atoms,provided that the total number of carbon atoms in the radicals R.sup.4 and R.sup.5 is at least 5,mineral oils containing one or more of said novel dyes and the use of said dyes for marking mineral oils.