Method of marking hydrocarbon liquids

Method of marking a hydrocarbon liquid includes adding to the liquid, a tracer compound of Formula I or II: wherein at least one of R 1 -R 6 in Formula I and at least one of R 7 -R 14 in Formula II is selected from: i. a bromine or fluorine atom; ii. a partially or fully halogenated alkyl group; iii. a branched or cyclic C 4 -C 20 alkyl group; iv. an aliphatic substituent linking two positions selected from R 1 -R 6 in Formula I to one another or two positions selected from R 7 -R 14 in Formula II to one another; or v. a phenyl group substituted with a halogen atom, an aliphatic group or halogenated aliphatic group  and none of R 1 -R 6 and none of R 7 -R 14 being a sulphonate group or COOR 15 , where R 15 represents H, C 1 -C 20 alkyl, C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, C 3 -C 15 cycloalkyl or aryl.