The trivalent lanthanide complex formation constants (log Kf) of the macrocyclic ligand H2ODO2A (4,10-dicarboxymethyl-1-oxa-4,7,10- triazacyclododecane) have been determined by pH titration techniques to be in the range 10.84-12.62 which increase with increasing lanthanide atomic number, and are smaller than those of the corresponding H2DO2A (1,7-dicarboxylmethyl-1,4,7,10-tetraazacyclododecane) complexes. The equilibrium formation of the dinuclear hydrolysis species, e.g. Ln2(ODO2A) 2(μ-OH)+ and Ln2(ODO2A)2(μ-OH) 2, dominates over the mononuclear species, e.g. LnODO2A(OH) and LnODO2A(OH)2-. Mass spectrometry confirmed the presence of [Eu(ODO2A)]+, [Eu(ODO2A)(OH)+H]+, [Eu2(ODO2A) 2(OH2)2+H]+, [Eu(ODO2A)(OH) 2]- and [Eu2(ODO2A)2(OH 2)3]- species at pH > 7. Density function theory (DFT) calculated structures of the EuODO2A(H2O) 3+ and EuDO2A(H2O)3+ complexes indicate that three inner-sphere coordinated water molecules are arranged in a meridional configuration, i.e. the 3 water molecules are on the same plane perpendicular to that of the basal N3O or N4 atoms. However, luminescence lifetime studies reveal that the EuODO2A + and TbODO2A+ complexes have 4.1 and 2.9 inner-sphere coordinated water molecules, respectively, indicating that other equilibrium species are also present for the EuODO2A+ complex. The respective emission spectral intensities and lifetimes at 615 nm (λex = 395 nm) and 544 nm (λex = 369 nm) of the EuODO2A+ and TbODO2A+ complexes increase with increasing pH, consistent with the formation of μ-OH-bridged dinuclear species at higher pH. Additional DFT calculations show that each Y(III) ion is 8-coordinated in the three possible cis-[Y2(ODO2A)2(μ-OH)(H2O)2] +, trans-[Y2(ODO2A)2(μ-OH)(H 2O)2]+ and [Y2(ODO2A) 2(μ-OH)2] dinuclear complex structures. The first and the second include 6-coordination by the ligand ODO2A2-, one by the bridged μ-OH ion and one by a water molecule. The third includes 6-coordination by the ligand ODO2A2- and two by the bridged μ-OH ions. The two inner-sphere coordinated water molecules in the cis- and trans-[Y2(ODO2A)2(μ-OH)(H2O) 2]+ dinuclear complexes are in a staggered conformation with torsional angles of 82.21° and 148.54°, respectively.