(U,Pu)O2 solid solutions are completely soluble in boiling 5 and 10 M nitric acid up to plutonium concentrations of 55 and 65 wt% of PuO2, respectively. The dissolution of the solid solutions decreases drastically at plutonium concentrations above 35 and 40 wt% of PuO2 in 5 and 10 M nitric acid, respectively, with a total metal content of 1 mol/ℓ. This is attributed to a saturation of the solution.