A method for marking a liquid petroleum hydrocarbon. The method comprises adding to the liquid petroleum hydrocarbon at least one substituted anthraquinone dye having formula (I) wherein R1 and R2 independently are hydrogen, hydroxy, OR11, amino or NR11R12; R3 and R5 independently are alkyl, aryl, aralkyl, heteroalkyl or heterocyclic; R4 and R6 independently are hydrogen or alkyl; R7, R8, R9 and R10 independently are cyano, nitro or hydrogen, provided that at least two of R7, R8, R9 and R10 are cyano or nitro; R11 is alkyl, aryl, aralkyl, heteroalkyl, heterocyclic or alkanoyl; R12 is hydrogen or alkyl; and wherein the substituted anthraquinone dye(s) has an absorption maximum in the range from 690 nm to 1000 nm.