The confluence of the rivers Negro and Solimões gives birth to the Amazon River near Manaus (Brazil). At their confluence, these two rivers with their very different physical and geochemical characteristics provide an interesting example of the mixing of waters along a reach of approximately one-hundred kilometres. The purpose of this article is to describe and explain the spatial configuration of the Solimões-Negro Rivers mixing zone, based on a systematic sampling survey carried out in September 1997, using a special sampler and an acoustic Doppler current profiler (Acoustic Doppler Current Profiler). The waters of the Negro River differ from those of the Solimões River in their lower velocity (0.3 vs 1 m s-1), conductivity (8 vs 80 μS cm-1 at 25° C), turbidity (5 vs 80 NTU), pH (5.5 vs 7.0) and higher temperatures (by 1 °C). Due to their higher density, the waters of the Solimões River slide under those of the Negro River, and consequently, Negro River waters are located at the surface, close to the left bank, and Solimões River waters are located at the bottom, close to the right bank. The contact between the waters of the two rivers changes from a clearly defined vertical boundary to a diffuse horizontal boundary, as they move downstream. The complete mixing process takes more than 30 h and 100 km. Copyright © 2009 John Wiley & Sons, Ltd.