As part of a continuing study, plutonium, americium, and neptunium speciation was determined at 25 and 90°C in four groundwaters from diverse sources: the Sparta aquifer in Louisiana, near the Vacherie salt dome; Mansfield No. 2 well in the Palo Duro Basin, Texas; the Stripa mine in Sweden; and the Waste Isolation Pilot Plant (WIPP) in New Mexico. Plutonium generally was soluble in Sparta and Stripa waters, regardless of temperature or initial oxidation state. Solubility in Mansfield water was high, except in 90 °C experiments using low-oxidation-state plutonium. The WIPP water had the least ability to maintain plutonium in solution; solubility after 30 days exceeded 50% only in experiments at 25 °C using Pu(V) and Pu(VI). Neptunium generally was soluble in all waters and was present exclusively as Np(V) and Np(VI), regardless of initial oxidation state. The solubility of americium was consistently high in Sparta groundwater at both temperatures and in Mansfield and WIPP waters at 25°C, but was < 50% after 30 days in Stripa water at both temperatures and in Mansfield and WIPP waters at 90°C. The results indicated that plutonium and neptunium solubilities were determined by the oxidation-reduction properties of the waters, i.e., their abilities to convert these elements to soluble oxidation states. This was not the case for americium, however; Am(IV) was not detected, and the solubility of this element was determined entirely by the chemical properties of Am(III).