Macrocyclic N,N′,N″,N″′,N″″,N″″′-hexakis(2-aminoethyl)-1,4,7,10,13,16-hexaazacyclooctadecane (haeh) and its complexes with Ni2+ and Cu2+ were synthesized. The protonation and formation constants with Ni2+, Cu2+, and Zn2+ were determined by pH-metry at 25 °C in a 0.5 mol dm−3 KNO3 aqueous solution. The macrocycle haeh forms stable di- and trinuclear species with Cu2+; log K(Cu2(haeh)) = 38.84 ± 0.04 and log K(Cu3(haeh)) = 49.08 ± 0.05. For Ni2+ and Zn2+, the macrocycle forms only a dinuclear complex; log K(Ni2(haeh)) = 29.00 ± 0.02 and log K(Zn2(haeh)) = 26.38 ± 0.02. These metal complexes show a significant tendency to form protonated and hydroxo species. The formation constants obtained are discussed on the basis of electrostatic repulsion and the flexibility of the macrocycles. The formation constants of the metal complexes were also determined at 15 and 35 °C in a 0.5 mol dm−3 aqueous KNO3 solution to obtain the thermodynamic parameters ΔH and ΔS. The results obtained showed that the stability of the copper complexes is dominated by ΔS.The synthesis and characterization of a new, fully functionalized hexaaza macrocycle ligand are described. The complexation of transition metal ions was investigated by means of potentiometric titration, and the estimated structure of the complexes formed with metal ions is discussed.