A rapid discrimination of counterfeit gas oils prepared by mixing fuel oil A with sulfuric acid (95% H2SO4) was studied by gas chromatography-atomic emission detection (GC-AED) and gas chromatography-mass spectrometry (GC-MS). Sulfuric acid is used for the removal of coumarin as an oil marker. Both the aromatic and sulfur contents in the counterfeit gas oils are reduced by sulfonation. The counterfeit gas oils gradually changed into oils with abundant paraffins. The GC-MS analysis showed no or a slight amount of the condensed polycyclic aromatic compounds in the counterfeit gas oils before the sulfuric content in the oils was reduced to 50 ppm or less on the basis of the sulfur analysis by GC-AED. These sulfur values corresponded to the sulfur content in the authentic gas oils from Japan. Therefore, the combination of GC-MS and GC-AED analyzed the composition of the aromatic compounds and the sulfur content in the oils and rapidly distinguished the counterfeit gas oils from the authentic gas oil.