A new alkylated quinizarin, 2(2-ethylhexyl) quinizarin, useful for marking water immiscible organic liquids; marked water immiscible organic liquids; and a method of marking said liquids with an additive which may be subsequently extracted from said liquid and easily identified by visual inspection. The method comprises dissolving in the liquid to be marked an additive selected from the group consisting of an alkylated quinizarin, the reaction product of diazotized ortho tolidine and pnonylphenol and of diazotized xylidine and p-nonylphenol. The additive is detected by extraction from the marked liquid by means of a water soluble polar organic solvent containing a base and from 0 to about 75 percent by weight of water to yield a distinctively colored extract which is identified by visual inspection. The identification may be further corroborated by chromatographic separation of the additive or by spectrophotometric analysis.