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A series of unknown cyclic compounds was detected in the capillary GC-MS analyses of extracts of marine sediments from the Tunisian coast at the Ž . Mediterranean sea. Using capillary GC-atomic emission detection AED , sulphur was elucidated in their structure. This allowed identification of the solutes as the thiosteranes 5-␣-cholestane-3-␤-thiol, 24-methyl-5-␣-cholestane-3-␤-thiol, and 24-ethyl-5-␣-cholestane-3-␤-thiol. These sulphur compounds are formed from ⌬-2 sterenes and probably originate from plankton. The presence of these thiols in coastal sediments varies considerably with the location of the sampling points, indicating that they are potential markers for environmental pollution.