Abstract: A method for tagging an oil so that subsequently it may be identified as to source by incorporating into the oil as a tracer material an oil-soluble or oil-dispersible Group VIB, Group VIIB or lanthanum series rare earth metal salt of a fatty acid. The so-tagged oil may be utilized in any desired manner. Thereafter, an oil suspected to contain at least a portion of the so-tagged oil is analyzed for the presence of the added metal moiety. The method is especially useful in tracing a complex oleaginous and petroliferous material so that it may be recovered and identified following a theft or spill or to trace the flow of such oil injected through a subterranean reservoir.