The hydrothermal system in Guaymas Basin, Gulf of California, converts immature organic matter in the overlying sedimentary cover to petroleum (methane to asphalt). This petroleum is scavenged by the fluids and thus migrates upward. The more volatile and soluble hydrocarbons (<C 10) brought to the seabed are injected into the ambient seawater by the hydrothermal circulation, whereas the heavy ends (>C 15) solidify in the mineral mounds at the sea floor. Such volatile and dissolved petroliferous material was sampled and analyzed in terms of its composition and boiling ranges to evaluate its sources, genetic processes and temperatures of both formation and migration. Interstitial gases in sediments of Deep Sea Drilling Project cores consisted of microbial methane mixed with thermogenic C 1-C 5 hydrocarbons at increasing subbottom depths. The water and surface sediments and mounds (young hydrothermal systems) all contained large concentrations of C 1-C 10 hydrocarbons (condensates). These are the volatile components missing in most of the mound samples (i.e. old, established systems). One plume sampled from a transform fault region consisted of natural gas (C 1-C 5 hydrocarbons). All gas and volatile samples had C 1 ( C 2- C 5) concentration ratios typical for a thermogenic origin (< 500) and contained varying amounts of aromatic hydrocarbons, primarily benzene and toluene, with minor amounts of ethylbenzene and xylenes. Headspace gases of the ambient waters contained relatively larger percentages of the aromatic volatiles when compared with those of the sediments, which is consistent with the higher aqueous solubilities of aromatics. Thus, the immature sedimentary organic matter (derived mainly from microbial and diatomaceous marine detritus) is easily pyrolyzed to aliphatic and aromatic products at subbottom depths and differentially transported and deposited by the hydrothermal fluids at the seabed. Post-depositional alteration by solubilization (water-washing) leads to continued scavenging of the more volatile and soluble components from the exposed hydrothermal deposits. Additional alteration by biodegradation leads to selective removal of the normal-, iso- and anteiso-alkanes and relative enhancements of the cycloalkanes or naphthenic components.