Solution equilibria of the ternary systems involving La(III), Y(III), Ce(III), and UO22+, N-(2-acetamido)iminodiacetic acid, and some aliphatic or aromatic acids have been investigated potentiometrically The formation of 1:1:1 mixed ligand complexes is inferred from the potentiometric titration curves. Formation constants of the different binary and ternary complexes formed in such systems were determined at 25 +/- 0.1-degrees-C and mu = 0.1 mol dm-3 (KNO3). It is deduced that the mixed ligand complexes are more stable than the corresponding binary complexes containing the aliphatic or aromatic acidate moiety. The order of stability of the binary and ternary complexes is investigated and discussed in terms of the nature of both the metal ion and the secondary ligand (aliphatic or aromatic acid).