The Amazon basin is the largest river catchment area in the world. It contributes about 20% of the world\u27s fluvial hydrological input. Samplings were done during low water levels (March 1995) on one tributary of the Amazon, the Rio Negro. Sequential tangential ultrafiltration (STUF) was used to separate total organic matter. The cut-off values were at 0.22 μm, 100 kD and 5 kD, while the organic pool separations were done for particulate, colloidal and dissolved material. Total organic carbon (TOC) fraction-size analysis was done for each group. Copper complexing capacity measurements were done by fluorescence quenching. Fluorescence intensities were measured on site. The results show that particulate and colloidal organic carbon are the predominant TOC forms in the black water river. In the case of the white water river, the dissolved fraction is the predominant organic carbon form. The complexing capacity for the Rio Negro waters is supported by the dissolved molecules, with some contribution from the particulate and dissolved fractions. Similar behaviour was observed for the white river water. The correlation observed between TOC and the fluorescence intensity shows an apparent quantum yield varying between 43 and 104 mKalle/ppm of carbon for the contribution of black waters to the total organic carbon. (1 Kalle = 0.1 mg of quinine sulphate per litre of 5 mM H2SO4.) Electromobility of the organic material is about 1 μS cm-1 per ppm of carbon. In mixing area studies, association of STUF with fluorescence intensity measurements provide relevant information for understanding the phenomena occurring in these areas
