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Abstract The solubility of Pu(IV) hydrous oxide is studied at 22±2 °C under Ar atmosphere in 0.25 and 3.5 M MgCl2 (pHc≈9) and 3.5 M CaCl2 (pHc=11–12) solutions in contact with iron powder. In alkaline 3.5–4.0 M CaCl2 ternary Ca-Pu-OH complexes are formed and the solubility of PuO2·xH2O(s) increases from about 10-10 M at pHc=11 to 10-8–10-7 M at pHc=12, both in the presence and absence of Fe powder. In the presence of iron the aqueous speciation is dominated by Pu(III) at pHc<10 and by Pu(IV) complexes at pHc>10. In the absence of reducing agents the aqueous speciation is dominated by Pu(V), with significant contributions of Pu(IV) complexes at pHc>10. The effect of ternary Ca-Pu(III,IV,V)-OH complexes in alkaline CaCl2 solutions can be described by assuming complex stoichiometries and experimental data from the oxidation state analogs Cm(III)/Nd(III), Th(IV) and Np(V). The ion interaction Pitzer approach is used for model calculations.