The chemical composition of liquid hydrocarbon fuels that affect engine combustion performance has historically been defined by three principal classes; saturates, monocyclic aromatics and dicyclic aromatics. These classes may be separated by normal phase analytical scale high performance liquid chromatography and are conveniently displayed in a chromatogram as the response of a differential refractive index (DRI) detector. Obtaining accurate quantitative analysis is not routine because the detector has a different response factor for each class. This response factor is proportional to the refractive index differential between the mobile phase and the analyte producing the signal. However, the detector response does vary linearly with the refractive index of the analyte when the volume of the injected sample is maintained constant. The problems associated with quantitation are overcome by obtaining two chromatograms of each fuel sample using two mobile phases which have different refractive indices. This eliminates the need to know the analyte refractive index before analysis. The method is applied to jet fuels in this work, but the extension to other hydrocarbon fuel types such as gasoline or diesel fuels is straightforward.