The speciation of plutonium, neptunium, and americium was determined in groundwaters from four sources in the Basin and Range Province: the lower carbonate aquifer, Nevada Test Site (NTS) (Crystal Pool); alluvial fill, Frenchman Flat, NTS (well 5C); Hualapai Valley, Arizona (Red Lake south well); and Tularosa Basin, New Mexico (Rentfrow well). Plutonium generally was most soluble in Rentfrow and well 5C waters and was present primarily in the higher oxidation states in these waters. Solubility was lowest in Crystal Pool water, apparently because this water was capable of reducing plutonium to relatively insoluble Pu(IV). In general, plutonium was comparatively insoluble in Red Lake south well water, but results were somewhat more variable. The behavior of neptunium paralleled that of plutonium, although the influence of oxidation-reduction properties of the groundwaters appeared to be less pronounced. The americium results were different from those of plutonium and neptunium, as would be expected from its existence in these waters solely as the trivalent ion. In general, it was relatively soluble in all waters at 25 °C, and relatively insoluble at 90°C. The results were interpreted to indicate that plutonium and, to a lesser extent, neptunium are least soluble in reducing groundwaters containing a large concentration of sulfate ion and a small concentration of strongly complexing anions. The results further emphasize the desirability of including studies such as this among the other site-selection criteria for nuclear waste repositories.