System for marking a hydrocarbon fluid to enable its authentication, the hydrocarbon fluid flowing from a source to a destination, the system comprising: a primary marker source, containing at least one primary marker; characterized in that the system further comprises a secondary marker source, containing at least one secondary marker which is different from said at least one primary marker, said secondary marker source coupled with said primary marker source; at least one sensor, operative for continuously measuring a fluid value of at least one fluid property of said hydrocarbon fluid; at least one fluid flow controller, coupled with said primary marker source and with said secondary marker source, a respective one of said fluid flow controller operative to admit a selected amount of said secondary marker into said primary marker source, and a respective one of said fluid flow controller operative to admit a selected amount of said primary marker containing said secondary marker into said hydrocarbon fluid; and a processor, coupled with said at least one sensor and with said at least one fluid flow controller, said processor operative to determine the selected amounts of said primary marker and said secondary marker according to said fluid value and according to predetermined concentrations of said primary marker and said secondary marker in said hydrocarbon fluid at said destination, such that the concentration of said secondary marker in said hydrocarbon fluid is substantially lower than the concentration of said primary marker in said hydrocarbon fluid, and such that the concentration of said primary marker in said hydrocarbon fluid is detectable by a first marker detector (302, 344), while the presence of said secondary marker in said hydrocarbon fluid is undetectable by said first marker detector (302, 344) but is detectable by a second marker detector (302, 344), said processor further operative to direct said fluid flow controller to admit said selected amount of said primary marker containing said secondary marker into said hydrocarbon fluid while flowing, marking and maintaining said hydrocarbon fluid at said predetermined concentrations of said primary marker and said secondary marker, and enabling the determination of the authenticity of said hydrocarbon fluid. a primary marker source, containing at least one primary marker; characterized in that the system further comprises a secondary marker source, containing at least one secondary marker which is different from said at least one primary marker, said secondary marker source coupled with said primary marker source; at least one sensor, operative for continuously measuring a fluid value of at least one fluid property of said hydrocarbon fluid; at least one fluid flow controller, coupled with said primary marker source and with said secondary marker source, a respective one of said fluid flow controller operative to admit a selected amount of said secondary marker into said primary marker source, and a respective one of said fluid flow controller operative to admit a selected amount of said primary marker containing said secondary marker into said hydrocarbon fluid; and a processor, coupled with said at least one sensor and with said at least one fluid flow controller, said processor operative to determine the selected amounts of said primary marker and said secondary marker according to said fluid value and according to predetermined concentrations of said primary marker and said secondary marker in said hydrocarbon fluid at said destination, such that the concentration of said secondary marker in said hydrocarbon fluid is substantially lower than the concentration of said primary marker in said hydrocarbon fluid, and such that the concentration of said primary marker in said hydrocarbon fluid is detectable by a first marker detector (302, 344), while the presence of said secondary marker in said hydrocarbon fluid is undetectable by said first marker detector (302, 344) but is detectable by a second marker detector (302, 344), said processor further operative to direct said fluid flow controller to admit said selected amount of said primary marker containing said secondary marker into said hydrocarbon fluid while flowing, marking and maintaining said hydrocarbon fluid at said predetermined concentrations of said primary marker and said secondary marker, and enabling the determination of the authenticity of said hydrocarbon fluid.