The solubility of plutonium hydrous oxide (PuO2(am, hyd)) was investigated in the presence of carbonate in neutral to alkaline, dilute to concentrated sodium nitrate (NaNO3) solutions. The observed dependence of the Pu(IV) solubility in low to medium concentration NaNO3 agreed with previously reported data and was well reproduced by the thermodynamic model curves, containing the species Pu(CO3)(4)(4-), Pu(CO3)(5)(6-), and Pu(OH)(2)(CO3)(2)(2-). Significant deviations between experimental and modeled Pu(IV) solubility were, however, observed for 2 mol.dm(-3) NaNO3 solutions. A more comprehensive chemical model including the complex Pu(OH)(2)(CO3)(3)(4-) was established, that reproduces the experimental results reasonably well. The conditional complex formation constants for Pu(OH)(2)(CO3)(2)(4-) were extrapolated to I = 0 using the specific ion interaction theory (SIT) and Pitzer approaches, and the corresponding ion interaction parameters were evaluated.