Volatile organic compounds (VOC) were quantified and identified at three petroleum seepage sites on the northern Gulf of Mexico continental slope. Concentrations of individual VOC ranged from below detection limit (0.01 ng/g) to greater than 500 ng/g with total concentrations as high as 7600 ng/g. Gas chromatographic patterns from these sites display a complex mixture of hydrocarbons, up to 400 individual VOC, and an unresolved complex mixture. This is in contrast to nonseep sites where relatively simple VOC mixtures are found (fewer than forty compounds) with individual concentrations less than 1.0 ng/g and a total concentration of less than 100 ng/g. VOC concentrations in sediments are dependent upon the amount and type of organic matter delivered to the sediment and the subsequent degradation process occurring in the sedimentary column.