Using benzaldehydes for marking hydrocarbons

The invention relates to the use of benzaldehydes of the formula (I), in which ring A can be benzo-anellated and R , R and R are hydrogen, hydroxy, C1-C15 alkyl, C1-C15 alkoxy, cyano, nitro or a radical of the formula NR R or COOR , in which R is hydrogen or possibly substituted C1-C15 alkyl, R is possibly substituted C1-C15 alkyl or a radical of the formula L-NX X , in which L is C2-C8 alkylene and X and X are mutually independently C1-C6 alkyl or, together with the nitrogen atom bonding them, a heterocyclic radical, and R is hydrogen, possibly substituted C1-C15 alkyl or a radical of the formula L-NX X , in which L, X and X have the above meanings, as marking agents for hydrocarbons, a process for detecting these benzaldehydes in hydrocarbons and hydrocarbons containing the above benzaldehydes.