Bottom sediments, fluid mud and suspended solids and dissolved mineral carbon were sampled during 8 cruises at dates corresponding to various tidal and hydrological conditions. Evaluation of the organic matter sources in particles and surface sediments into the Loire Estuary has been performed using molecular markers. The results obtained show that the natural organic matter is an admixture of terrigenous and algal components. The terrigenous signature derived from land plants is present in the whole estuary and reflect a uniform dispersal of terrestrial inputs. The production of biogenic material by aquatic photosynthetic organisms is higher in the fluvio-estuarine zone and decreases seaward. In addition to natural organic matter, a contamination from fossil fuels and components derived from pyrolysis has been assessed. Organic carbon in suspended matter and sediments show homogenous ¹⁴Cvalues (0.3± 0.2Bq/g) confirming a soil origin. At the opposite, ¹⁴C activity of dissolved mineral carbon is higher ranging between 150 to 200% of modern carbon activity and clearly indicates a noticeable contribution from reactors. Radiocarbon concentrations decrease seaward and are strongly correlated with salinity reflecting hydrodynamic and stratification processes in the marine estuary.