C, N, P and Si in dissolved, particulate, organic and inorganic forms, determined from 12 surveys in the river Oitabén-Verdugo during 2002, allowed quantification of the chemical composition, phase partitioning and fluxes of continental materials to the inner Ría de Vigo (NW Spain). Despite the large fluxes normalised to the drainage basin of the river Oitabén-Verdugo (280mmol Cm⁻²y⁻¹, 56.5mmol Nm⁻²y⁻¹, 1.35mmol Pm⁻²y⁻¹, and 61.3mmol Sim⁻²y⁻¹), its reduced dimensions (350km²) yielded relatively low concentrations of these species. Organic C, N and P represented 62%, 43% and 48% of the total C, N and P fluxes, respectively. About 90% of the organic C and N and 60% of the organic P was in the dissolved form. The chemical composition of the organic materials was compatible with detrital organic matter derived from aquatic plants, phytoplankton and soil leaching but not with unprocessed terrestrial plants. Significant temporal changes were observed in the chemical composition, related to the seasonal cycle of the biological activity in stream waters and soils, as well as the chemical weathering reactions. Spatial differences were due to the contrasting dynamic regime and, likely, the origin of the materials.