Mixed-ligand complexes of Co-II, Ni-II, and Cu-II with dicarboxylic amino acids (aspartic and glutamic or H(2)ADA) as primary ligands and 8-hydroxyquinoline as a secondary ligand were prepared and characterized. Formation constants of the different complexes were determined acidimetrically at 25 degrees C and at ionic strength I = 0.1 mol dm(-3) (KNO3) and were discussed in terms of the nature of both the metal ion and the ligand used. The thermodynamic functions (Delta H, Delta G degrees, and Delta S degrees) associated with the formation of [M-II + (HADA)(1-)+ 8-hydroxyquinoline] complexes were also calculated and discussed.