Solubility of UO2, NpO2, and PuO2 powders in simulated groundwater (SGW) was studied by attainment of the equilibrium from below. The solubility was determined in AnO2-SGW heterogeneous systems at different solid: liquid weight ratios and different pH values of the examined solutions both under ambient conditions and in an inert atmosphere. The solubility of UO2, NpO2, and PuO2 depends on the solid: liquid ratio but is independent of pH of the solutions in the range from 6 to 11. The solubilities of UO2 and NpO2 in SGW under ambient conditions are higher that those in an inert atmosphere. The solubility of freshly calcined PuO2 is virtually independent of the atmosphere. Formation of amorphous hydroxides by oxidation of the dioxide surface, whose area is proportional to the weight of the dioxides, increases their solubility.