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A systematic characterization of the geochemistry of rhenium was undertaken. Three regimes were studied: (1) a major river basin (the Orinoco), (2) a large estuary (the Amazon), and (3) the open ocean (Atlantic and Pacific). In 29 rivers throughout the Orinoco basin, Re concentrations were found to vary by more than 4 orders of magnitude, from <0.02 pmol/kg to 412 pmol/kg. The highest values were observed in the Portugueza Basin, a region in the southeastern Andean foothills containing substantial black shale deposits. The lowest values were measured in tributaries draining the intensely-weathered Precambrian Guyana Shield on the right bank of the Orinoco. Measurements of dissolved Re from 13 locations in a transect of the Amazon estuary fall on or close to the conservative mixing line, although a positive deviation below 5 per mil salinity suggests the existence of a Re source on the inner shelf