Preparative pyrolysis followed by liquid chromatographic separation and gas chromatography of hydrocarbons (Pyrolysis-LC-GC) were used to study the participation of terrestrial and/or aquatic compounds in the formation of humic substances in sediments. Analyses performed on humic substances from recent sediments sampled in the Elorn river estuary (France) show that these substances are similar in many respects to other lacustrine or marine humic substances, with regard, for instance, to their high nitrogen and sulphur content and their low aromaticity. Mass balances on pyrolysis products and group composition of the C-14+ fraction of the pyrolysates were performed. The distribution of the total alkane and alkene fraction, isocyclo alkanes and alkenes from these humic acid pyrolysates was compared to that of various reference humic acid pyrolysates. The biomarker study (distribution and hopanes/steranes ratios from humic acid pyrolysates) confirms the participation throughout the estuary of algal precursors in the formation of the studied compounds, irrespective of the location of the sampling zone (upstream, median or downstream part of the estuary).