Mass-spectrometer analyses were made of gases extracted under vacuum from sediments of marshes and deep marine basins off southern California. The results show that gases other than oxygen, nitrogen, and argon are much more abundant in sediments than in the overlying water. Carbon dioxide and methane increase markedly with depth in the sediment, more or less paralleling the previously known increases of ammonia and hydrogen sulphide. Although data for them are less complete, ethane, butane, hexane plus, isopentane, cyclobutane, cyclopentane, benzene, and toluene also exhibit general increases with depth. Methane in sediments 124 to 148 inches deep at the bottom of the Santa Barbara Basin weighs nearly as much as the total of all non-volatile heavy hydrocarbons in the sediment, thus providing a gas-oi! ratio similar to that of oil fields. A very high methaneethane ratio of the sediment gases contrasts with the low ratios in oil fields and indicates that further diagenesis is required before the sediments gases are like those in oil fields.