A method of marking a hydrocarbon liquid selected from a diesel fuel and a gasoline fuel comprising the step of adding to said liquid, as a tracer compound, a compound of Formula I: wherein, each A is independently selected from the group consisting of (i) a phenyl group, (ii) a phenyl group substituted with one or more halogen atoms, an aliphatic group or a halogenated aliphatic group, (iii) a partially or fully halogenated alkyl group or (iv) a linear branched or cyclic C 1 - C 20 alkyl group, and each B is independently selected from the group consisting of a (i) phenyl, (ii) an unsubstituted phenylmethyl group, (iii) a substituted phenyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, (iv) a substituted phenylmethyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, or (v) 1,1-dimethylpropyl, 1,1-dimethylbutyl, 1-ethyl-1-methylpropyl, 2,2-dimethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-2,2-dimethylpropyl, 1-methylethyl-2,2-dimethylpropyl, 2-methylcyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, cyclohexylmethyl, 1-adamantyl, 2-adamantyl and decahydronaphthyl groups; wherein said tracer compound is added to said hydrocarbon liquid at a concentration within the range from 1 microgram per litre to 1000 micrograms per litre. each A is independently selected from the group consisting of (i) a phenyl group, (ii) a phenyl group substituted with one or more halogen atoms, an aliphatic group or a halogenated aliphatic group, (iii) a partially or fully halogenated alkyl group or (iv) a linear branched or cyclic C 1 - C 20 alkyl group, and each B is independently selected from the group consisting of a (i) phenyl, (ii) an unsubstituted phenylmethyl group, (iii) a substituted phenyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, (iv) a substituted phenylmethyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, or (v) 1,1-dimethylpropyl, 1,1-dimethylbutyl, 1-ethyl-1-methylpropyl, 2,2-dimethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-2,2-dimethylpropyl, 1-methylethyl-2,2-dimethylpropyl, 2-methylcyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, cyclohexylmethyl, 1-adamantyl, 2-adamantyl and decahydronaphthyl groups; (i) phenyl, (ii) an unsubstituted phenylmethyl group, (iii) a substituted phenyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, (iv) a substituted phenylmethyl group in which the benzene ring is substituted by at least one of the substituents selected from the group consisting of a fluorine atom, a partially or fully halogenated alkyl group and a linear, branched or cyclic C 1 - C 20 alkyl group, or (v) 1,1-dimethylpropyl, 1,1-dimethylbutyl, 1-ethyl-1-methylpropyl, 2,2-dimethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-2,2-dimethylpropyl, 1-methylethyl-2,2-dimethylpropyl, 2-methylcyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, cyclohexylmethyl, 1-adamantyl, 2-adamantyl and decahydronaphthyl groups; <EOS>