A method for marking a petroleum hydrocarbon or a liquid biologically derived fuel; said method comprising adding to said petroleum hydrocarbon or liquid biologically derived fuel at least one compound that is a R1, R2, R3, R4, R5, R6, R7, R8, R9 and R10-substituted xanthene, wherein R1, R2, R3, R4, R5, R6, R7, R8, R9 and R10 independently are hydrogen, hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; wherein each compound of formula (I) is present at a level from 0.01 ppm to 20 ppm.