[en] Radionuclide concentrations of a number of elements (Am, Pu, U, Pa, Th, Ac, Ra, Po, Pb, Cs, and Sr) have been measured in the water and sediments of a group of alkaline lakes in the western USA. These data demonstrate greatly enhanced soluble phase concentrations of elements with oxidation states of III, IV, V, and VI as the result of carbonate complexing. Dissolved concentrations of isotopes of U, Pa, and Th in a lake with pH = 10 and a total inorganic carbon concentration of 4 x 10^-1 moles/1 were greater than those in sea water (pH = 8, ΣCO₂ = 2 x 10^-3 moles/1) by order of magnitude for ²³³U, ²³⁸U (--10²), ²³¹Pa, ²²⁸Th, ²³⁰Th (--10³) and ²²Th (--10⁵). Concentrations of fallout /sup 239,240/Pu in the more alkaline lakes were equivalent to effective distribution coefficients of --10³, about a factor of 10² lower than in most other natural lakes, rivers, estuaries and coastal marine waters. Measurements of radionuclides in natural systems are essential for assessment of the likely fate of radionuclides which may be released from high level waste repositories to ground water. Laboratory-scale experiments using tracer additions of radionuclides to mixtures of water and sediment yielded distribution coefficients which were significantly different from those derived from field measurements (10¹-10² lower for Po and Pu). Order of magnitude calculations from thermodynamic data of expected maximum U and Th concentrations, limited by pure phase solubilities, suggest that carbonate complexing can enhance solubility by many orders of magnitude in natural waters, even at relatively low carbonate ion concentrations