Methane carbon-isotopic compositions (δ1 3C values relative to the PDB standard) at Sites 565, 566, 567, and 569 were lighter (enriched in 12C) than -60‰, indicating a biogenic origin. In the deeper sections at Sites 568 and 570, δ1 3C values were heavier, approaching - 40‰, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide δ1 3C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. δ1 3C values of ethane and higher hydrocarbons were measured in several sections from Sites 566 and 570 that contained sufficient C2-C4 hydrocarbon concentrations. Ethane values in six sections (245-395 m sub-bottom) from Site 570 were fairly uniform, ranging from - 24 to - 26%o. These values are among the heaviest ethane values reported for natural gases. The isobutane/n-butane and isopentane/«-pentane ratios of the core gases suggested that the C2-C5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter. This process apparently generates isotopically heavy C2-C5 hydrocarbons. High gas concentrations in the serpentinite basement rocks at Sites 566 and 570 appear to have resulted from migrated biogenic methane gas containing small amounts of immature C2-C5 hydrocarbons.