Abstract Known data on the solubility of Am(OH)₃(s) and the hydrolysis of Am(III) and Cm(III), additional information from an extensive solubility study with Nd(OH)₃(s) in NaCl, MgCl₂, and CaCl₂ media of various ionic strengths and spectroscopic (time-resolved laser fluorescence spectroscopy, TRLFS) data for Cm(III) in alkaline CaCl₂ solutions are used to evaluate a comprehensive set of standard-state equilibrium constants and ion interaction parameters for the specific ion interaction theory (SIT) and Pitzer equations at 25 °C. The thermodynamic model takes into account the analogous solubility and hydrolysis behavior of trivalent actinides and Nd(III) and covers the entire pH range in dilute to concentrated NaCl, MgCl₂, and CaCl₂ solutions. In alkali chloride/hydroxide solutions, the formation of the tetrahydroxide complex M(OH)₄ ^– requires OH^– concentration above 3 mol l^–1, whereas in alkaline CaCl₂ solutions (at pH_c < 12) M(III) complexes with four and six hydroxide ligands are formed. Similar as the recently detected ternary Ca–M(IV)–OH complexes Ca₃[Zr(OH)₆]⁴⁺ and Ca₄[Th(OH)₈]⁴⁺, these complexes are stabilized by the association of Ca²⁺ ions. The solubility and hydrolysis of Am(III), Cm(III), and Nd(III) in both Ca-free and -containing solutions is consistently described with a model including the ternary Ca–M(III)–OH complexes Ca[M(OH)₃]²⁺, Ca₂[M(OH)₄]³⁺, and Ca₃[M(OH)₆]³⁺.