A process for detecting marked mineral oils by treating the marked mineral oil with an extracting agent containing water, a solvent and a base, in which use is made as the marking agent of an azo dye of formula (I), in which ring A may be benzo-anellated and l is 1 or 2, m is 1 to 4, X1 is hydrogen, C¿1?-C4-alkyl, cyano, nitro or possibly substituted phenylazo; X?2¿ is hydrogen, C¿1?-C4-alkyl, cyano, nitro, C1-C4-alkoxy or C1-C16-alkoxy carbonyl; X?3¿ is hydrogen, C¿1?-C4-alkyl, cyano or C1-C16-alkoxy carbonyl and X?4¿ is hydrogen, hydroxy, possibly substituted C¿1?-C8-alkyl, C1-C4-alkoxy, amino, C1-C4-dialkyl amino or possibly substituted C1-C16-monoalkylamino. A partially or completely water-miscible organic solvent is used as the solvent and the base is an alkaline or alkaline earth hydroxide, an alkali carbonate or a quaternary ammonium hydroxide. The invention also relates to the use of azo dyes as marking agents for mineral oils and novel azo dyes.