PURPOSE: To easily and precisely distinguish the brands and the grades of petroleum products using reactive extraction system by labeling petroleum products with the new silent markers.

CONSTITUTION: Firstly, markers of the formula is added to petroleum products in a detectable level, usually in an amount which results a concentration of 0.25 ppm or more in the final liquid petroleum product. Subsequently, the marker of the formula is extracted by an alkaline aqueous medium (suitably, a 50/50 mixture of a 2 wt.% NaOH aqueous solution and methanol) from the same petroleum product. Finally, the color of the marker in the extract liquid is observed. As the markers, for example, there is 2,6-di-sec-butyl-4-[2',5'- dimethoxy-4'-(4'-nitrophenylazo)]phenylazophenol or the like. In the formula, W is H or O-(1-3C) alkyl, provided that at least one of W is O-(1-3C) alkyl; X, Y are each H, alkenyl or the like.