Abstract: A method for determining the concentration of a tracer, e.g. nitrogen-bearing marker dye, in a hydrocarbon liquid, e.g. gasoline. In the present method, a sample of the marked hydrocarbon liquid which is to be analyzied is prepared by adding a known volume of a nitrogen-bearing, "internal standard" thereto. The sample is then heated, preferably in a gas chromatograph, to vaporize, separate, and measure the dye and the internal standard. These measurements provide a ratio which is matched or compared to a predetermined, reference ratio which, in turn, is representative of a known concentration of the dye in the sampled hydrocarbon.