The formation of ternary Mg-UO2-CO3 complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO2-CO3 complexes. The presence of two different Mg-UO2-CO3 complexes was identified by means of two distinct fluorescence lifetimes of 17 +/- 2 ns and 51 +/- 2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO2-CO3 complexes forming. For the first time, the formation of both MgUO2(CO (3))(3)(2-) and Mg2UO2(CO3)(3) (aq) species was confirmed and the corresponding equilibrium constants were determined as log beta(0)(113) = 25.8 +/- 0.3 and log beta(0)(213) = 27.1 +/- 0.6, respectively. Complementarily, fundamental structural information for both Ca-UO2-CO3 and Mg-UO2-CO3 complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 angstrom) compared with U-Ca distance (4.15 angstrom). These results confirmed the inner-sphere character of the Ca/Mg-UO2-CO3 complexes. The formation constants determined for MgUO(CO3)(3)(2-) and Mg2UO2 (CO3)(3) (aq) species indicate that ternary Mg-UO2-CO3 complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.