Stable and radioactive carbon isotopic compositions of particle size fractions of a surface sediment from the Ems-Dollard estuary vary considerably with particle size. The organic material in the fine fractions (<20 μm) has considerably higher 14C values (14a∼80%) than that in the coarse fractions (52%) and has higher δ13C values (average of −23‰ and −25.6‰, respectively). This shows that OM in the fine and the coarse fractions has different sources. The organic carbon in the fractions with particle sizes <20 μm is mainly imported from the North Sea. The contribution of material from the Ems river appears negligible. The carbon isotopic composition of the coarse fractions points to a terrestrial contribution. Discrete organic fragments are found of both terrestrial and marine/estuarine origin.