AbstractThe solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22±2 °C under reducing conditions in alkaline CaCl₂solutions. Redox conditions were adjusted either with 2 mM Na₂S₂O₄or additions of iron powder. In 2.0 and 4.5 M CaCl₂and pH_c=11–12, the neptunium and plutonium concentrations increase with a slope of +4 (log[An]vs. pH_c) as expected for the formation of the complex Ca₄[An(OH)₈]⁴⁺recently identified for Th(IV). At CaCl₂concentrations ≤1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log^*K_s,(4,1,8)for the reactionAn(OH)₄(am) + 4 H₂O + 4 Ca²⁺⇔ Ca₄[An(OH)₈]⁴⁺+ 4 H⁺are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5–4.5 M CaCl₂. The log^*K°_s,(4,1,8)values and the complex formation constants logβ°_(4,1,8)follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).