An identification method which is a method of identifying a known marked product in an unknown sample, the identification method comprising: producing said known marked product, being a product having a distillation range, by adding to said product a first marker, a second marker and optionally one or more further markers, each marker having a distillation range including a minimum boiling point (Min BP) and a maximum boiling point (Max BP) which are the minimum and maximum temperatures of each marker's distillation range, and each marker having a maximum distillation boiling point (Max DBP) which is the temperature at which the maximum volume of the marker distils being the temperature at which a distillation profile of temperature vs concentration of marker in the distillate passes through a maximum, each marker having a different distillation range from each other marker and at least one marker having a Max DBP which is within the distillation range of the product wherein the first marker has a distillation range in the marked product which extends over the whole of the product distillation range; analysing said sample by an analytical method capable of providing a response which is proportional to the quantity of marker found in the sample to identify whether said first marker, said second marker or a said further marker is present in said unknown sample; calculating a response ratio of the first and second markers in the unknown sample, to each other, and comparing the response ratio of the first and second markers in the unknown sample with a response ratio of the same markers in a reference sample to determine whether the sample contains the known marked product or a distilled fraction or a residue fraction thereof. producing said known marked product, being a product having a distillation range, by adding to said product a first marker, a second marker and optionally one or more further markers, each marker having a distillation range including a minimum boiling point (Min BP) and a maximum boiling point (Max BP) which are the minimum and maximum temperatures of each marker's distillation range, and each marker having a maximum distillation boiling point (Max DBP) which is the temperature at which the maximum volume of the marker distils being the temperature at which a distillation profile of temperature vs concentration of marker in the distillate passes through a maximum, each marker having a different distillation range from each other marker and at least one marker having a Max DBP which is within the distillation range of the product wherein the first marker has a distillation range in the marked product which extends over the whole of the product distillation range; analysing said sample by an analytical method capable of providing a response which is proportional to the quantity of marker found in the sample to identify whether said first marker, said second marker or a said further marker is present in said unknown sample; calculating a response ratio of the first and second markers in the unknown sample, to each other, and comparing the response ratio of the first and second markers in the unknown sample with a response ratio of the same markers in a reference sample to determine whether the sample contains the known marked product or a distilled fraction or a residue fraction thereof.