[en] The acid dissociation constants, pK_a, and the stability constants for NpO⁺₂ and UO²⁺₂ have been measured for certain organic ligands [acetate, α-hydroxyisobutyrate, lactate, ascorbate, oxalate, citrate, EDTA, 8-hydroxyquinoline, 1,10-phenanthroline, and thenoyltrifluoroacetone] in 5 m (NaCl) ionic strength solution. The pK_a values were determined by potentiometry or spectrometry. These methods, as well as solvent extraction with ²³³U and ²³⁷Np radiotracers, were used to measure the stability constants of the 1:1 and 1:2 complexes of dioxo cations. These constants were used to estimate the concentrations required to result in 10% competition with hydrolysis in the 5 m NaCl solution. Such estimates are of value in assessing the solubility from radioactive waste of AnO⁺₂ and AnO²⁺₂ in brine solutions in contact with nuclear waste in a salt bed repository. (orig.)