Concentrations of interstitial methane in the sediment sequences at all five Leg 207 sites increase abruptly in the black shales, where they reach their maximum values. A weak correspondence exists between higher TOC and greater methane concentrations, but marked excursions from a simple linear relation suggest that the type of organic matter, and not simply the amount, affects gas generation in the black shales. Because of the low thermal maturity of organic matter at all five sites and the predominance of methane in the interstitial gases, the origin of most of the gases is likely from in situ microbial activity. Dramatic decreases in methane concentration at the tops of the black shale boundaries suggest that methane oxidation, which consumes interstitial sulfate, proceeds in overlying units. Evidence of migration of some hydrocarbon gases into the drilled sequences from deeper, more thermally mature sediments is reflected in the presence of small amounts of heavier hydrocarbon gases. Active generation of methane in the black shale units likely replaces gas that may migrate out of these lithostratigraphic units to maintain the elevated methane concentrations that exist within the black shales.