Application of trace-metal and biological markers to geochemical studies has shown that crude oils could be correlated or differentiated according to their geologic age. The V/Ni, V/Sigma Ni, Mg, Fe), and Pr/Ph markers were almost uniform in Gulf of Suez crude oils, revealing their same origin, yet showing marked differences in Western Desert crude oils, reflecting varying degrees of their maturity and migrational history. The significance of petroleum markers was extended to monitoring of oil spill sources. Weathering of spills usually renders their source identification questionable by infrared or gas chromatography profiles. Since evaporative loss light petroleum fractions does nor appreciably affect the high-Molecular Weight components with which trace metals, isoprenoids, hopanes, and steranes are associated, V/Ni, Pr/Ph, m/z 191, and m/z 217 mass chromatogram fragments; were found reliable in fingerprinting oil spill sources in Mediterranean waters.