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Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over-and undersaturation at defined CO 2 partial pressures. The solubility of 237 Np from oversaturation ranged from a high of (9.40 ( 1.22) × 10 -4 M at pH 6.0 and 60 °C to a low of (5.50 ( 1.97) × 10 -6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240 Pu solubilities ranged from (4.70 ( 1.13) × 10 -8 M at pH 6.0 and 25 °C to (3.62 ( 1.14) × 10 -9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenishbrown crystalline Np 2 O 5 ‚xH 2 O was identified as the solubility-limiting solid using X-ray diffraction. A mean thermodynamic solubility product for Np 2 O 5 ‚xH 2 O of log K°s p ) 5.2 ( 0.8 for the reaction Np 2 O 5 ‚xH 2 O + 2 H + h 2NpO 2 + + (x+1)H 2 O at 25 °C was calculated. Sparingly soluble Pu(IV) solids, PuO 2 ‚xH 2 O and/or amorphous plutonium(IV) hydroxide/colloids, control the solubility of plutonium in J-13 water.