A number of different hydrocarbon components were detected in sediments from Sites 1109 and 1115 using the purge-trap procedure, which were not detected during shipboard gas analysis. Considerable concentrations of volatile hydrocarbon components >C5 were also measured in the presence of very low concentrations of propane and, more importantly, where no propane was detected during routine shipboard headspace analysis. The LMWH downhole profiles plus low in situ temperatures suggest that these components are formed in situ by low-temperature biological processes rather than high-temperature thermogenic processes, which are usually thought to be involved. Purge-trap analysis has indicated the presence of some unexpected deep low-temperature bacterial reactions, which demonstrates that further analysis of LMWH may provide valuable information at future ODP sites.