The first study of the molecular size distribution and fluxes of organic carbon from the Rio Purus to the Amazon River is reported. Organic carbon measurements using sequential tangential ultrafiltration (STUF) and discharge measurements give original information for the Purus basin, which has not been studied in depth until now. For this basin, the total organic carbon (TOC) values and the molecular size distribution are similar to those found for white waters. This type of water is the most common in the Amazonian basin and is characterised by a high concentration of mineral material and a low content of organic matter. In addition two sampling sites of the Purus River show a very large concentration (>80% of the TOC concentration) in the dissolved material. This distribution is similar to that of the Rio Madeira, which is unexpected, since the Rios Purus and Madeira do not drain the same kind of soil. For the Amazon basin, this work has confirmed the carbon size redistribution and flocculation/coagulation phenomena at the confluence. This dynamic exchange between the fractions is important for some phenomena such as metals transported by the organic matter. Indeed the metal transport and the molecular mass distribution of associated metals to organic matter are closely linked to the size of the organic material. Copyright (C) 1999 Elsevier Science B.V.