A separation technique was developed for the characterization and identification of the nonaromatic fraction of a light and a heavy FCC feedstock (gas-oil). The separation of the paraffins was based upon the selective reaction of n-paraffins with urea and branched paraffins with thiourea. One more method based on the molecular sieves adsorption was also applied for n-paraffin separation, but the urea method was found to be more satisfactory. The total recovery of the method was about 70%. All the fractions were characterized using a system of GC/MS under the specific conditions presented in this work. The hydrocarbons in the n-paraffinic and isoparaffinic fractions were identified satisfactorily, while for the naphthenic fraction this identification was more difficult. A quantitative analysis was attempted of all the paraffinic compounds in the FCC feedstock. Using a standard sample of n-paraffins, the relative wt % concentrations of the n-paraffinic and isoparaffinic compounds were determined. For the cycloparaffins, the method of ASTM D2786 was applied and the concentrations of naphthenes mere approximately estimated according to the number of rings in these compounds.