Data for 226Ra and 228Ra in the Amazon River system show that the activity of each radium isotope is strongly correlated with barium concentrations. Two trends are apparent, one for rivers which drain shield areas and another for all other rivers. These data suggest that there has been extensive fractionation of U, Th, and Ba during weathering in the Amazon basin. The 226Ra data fit a flux model for the major ions indicating that 226Ra behaves conservatively along the main channel of the Amazon River.