The effects of imposed variations of pH and Eh on aqueous uranium mobility at 25°C have been studied in three simulations of natural water systems. Constituents tested for their effect on uranium mobility were: 1. (a) hydrous ferric oxide, to represent adsorptive solids which precipitate or dissolve in response to variations in pH and Eh. 2. (b) kaolinite, representing minerals which, although modified by pH and Eh changes, are present as solids over the pH-Eh range of natural waters. 3. (c) carbonate, to represent a strong uranium-complexing species. Uranium mobility measurements from each simulation were regressed against pH and Eh within a range appropriate to natural waters. Hydrous ferric oxide and kaolinite each affected uranium mobility, but in separate pH-Eh domains. Aqueous carbonate increased mobility of uranium, and adsorption of UO2(CO3)3 4- caused colloidal dispersion of hydrous ferric oxide, possibly explaining the presence of 'hydrothermal hematite' in some uranium deposits. Enhanced uranium mobility observed in the pH-Eh domains of thermodynamically insoluble uranium oxides could be explained if the oxides were present as colloids. Uranium persisting as a mobile species, even after reduction, has implications for the near surface genesis of uranium ores.