The distributions of beryllium-9 and beryllium-10 in rivers within the Orinoco and Amazon basins have been examined to extend our understanding of their geochemical cycles and to develop their use both in geochronometry, and in studying erosional processes. Beryllium-9, the stable isotope, is weathered from mineral lattices before entering the riverine dissolved or exchangeable phase. The cosmogenic radioisotope 10Be (t 1 2 = 1.5 Myr) is produced primarily in the atmosphere and is brought into riverine systems, via rainfall, in dissolved or exchangeable form; it may be used to examine the processes which affect beryllium partitioning between the dissolved and particulate phases. Ancillary data, such as major ion distributions, provide a basis for selecting regions in which riverine chemistry is dominated by a single geochemical process, allowing examination of its effects on Be distributions in isolation. Analyses of 9Be in dissolved and suspended material from rivers with a wide range of chemical compositions indicate that its geochemistry is primarily controlled by two major factors: 1. (1) its abundance in the rocks of the watershed and 2. (2) the extent of its adsorption onto particle surfaces. The relative importance of these parameters in individual rivers is determined by the extent of interaction with flood-plain sediments and the riverine pH. This understanding of 9Be geochemistry forms a basis for examination of the geochemical cycling of 10Be. In rivers which are dominated by interaction with sediments, the riverine concentration of dissolved 10Be is far lower than that in the incoming rainwater, indicating that a substantial proportion of it is retained within the soils of the basin or is adsorbed onto riverine particles. However, in acidic rivers in which the stable dissolved Be concentration is determined by the Be level in the rocks of the drainage basin, dissolved 10Be has essentially the same concentration as in precipitation. These observations imply that the soil column in such regions must be saturated with respect to 10Be, and that the ratio of the inventory to the flux does not represent an age, as may be the case in temperate latitudes, but rather a residence time.