Hydrothermal activity and seabed mounds have been explored in Guaymas Basin by the Deep Sea Drilling Project (DSDP), piston coring, dredging, and diving with the Deep Submersible Research Vessel (DSRV) Alvin. Sedimentary organic matter, derived primarily from immature, degraded microbial detritus, is easily converted to petroleum under the hydrothermal regime. These petroleums are mature and migrate in the fluids and by diffusion to the seabed. The fluid migration is aided by near-critical aqueous solution and supercritical carbon dioxide and hydrocarbons. Petroleum compositions vary from condensates to naphthenic to waxy, all with significant amounts of asphaltenes and hydrothermal products such as olefins and toxic polynuclear aromatic hydrocarbons (PAH). The heavy ends condense at the seabed, depositing mainly as a cement with the sulfides and other minerals and to a lesser extent as entrapped oil and crystalline wax in vugs and conduits of the mounds. The PAH are high-temperature resynthesis-aromatization products from residual organic matter, and they are present in all oils but also deposit as discrete trace fractions in the hottest regions of the vent systems. Preliminary estimates of total hydrocarbon generation during hydrothermal alteration indicate that this process has a significant petroleum potential.