The formation equilibria of new ternary mixed-ligand complexes of copper(II), nickel(II), cobalt(Il), and manganese(II) with N-(2-acetamidoiminodiacetic) acid (H2ADA) as the primary ligand and histidine (His) as the secondary one were investigated. The formation constants of these complexes have been determined at 25.0 °C in 0.3 M aqueous Na2SO4 solution. They show that the ternary complexes are more stable than the corresponding binary ones. The ternary complexes have been synthesized and were characterized using elemental analyses, molar conductance, 1R spectra, magnetic measurements as well as electronic and ESR spectra. The spectral results support the binding of H2ADA and histidine with, respectively, the NOO and NO sites, to copper(II), cobalt(II), and manganese(II) providing an octahedral geometry, while nickel(II) is bonded to the same ligands in a bidentate manner by the OO and NO donor groups giving a square-pyramidal complex. The other axial sites are occupied by OH− or H2O groups. Magnetic and ESR data indicate that cobalt(II) and manganese(II) adopt a high-spin configuration in the complexes and also show that the metal ions do not undergo any change in the oxidation state.