Abstract The evolution of the sulphur content of humic acids extracted from a near-shore sediment core was investigated. Special attention was taken to avoid S contamination of the humic materials during extraction. The S C ratios increased continuously with depth to values which strongly suggest S addition to the humic matrix during early diagenesis by reactions between organic matter and H 2S or its oxidation products. Their light isotopic composition supports this view, however, subsequent isotopic exchange has obscured the mechanism initially involved. Since a large fraction of the enrichment occurred above the sulphidic zone, redox boundaries, such as the interface of anoxic microniches within more oxidized zones, or the sulphidic/suboxic boundary of the sediment column, must have been important sites for S addition. Bioturbation, by increasing the contact between sedimentary zones of a different redox regime, will likely enhance such processes.