The formation of and stability constants for aqueous Mg-UO2-CO3 complexes were determined using an anion exchange method. Magnesium concentration was varied (up to 20 mmol/L) at constant ionic strength (I = 0.101, 0.202, 0.304, 0.406, and 0.509 mol/kg NaNO3), pH 8.1, total [U(VI)] = 10.4 micromol/L under equilibrium with atmospheric CO2. The results indicate that only the MgUO2(CO3)3(2-) complex is formed. The cumulative formation constant extrapolated to zero ionic strength is similar regardless of the activity correction convention used: logbeta113(0) = 25.8 +/- 0.5 using the Davies equation and = 25.02 +/- 0.08 using specific ion interaction theory (SIT). Uranium sorption onto the exchange resin decreased in the presence of Mg putatively due to the formation of MgUO2(CO3)3(2-) that had a lower affinity for the resin than UO2(CO3)3(4-). Uranium sorption results are consistent with an equivalent anion exchange reaction between NO3- and UO2(CO3)3(4-) species to retain charge neutrality regardless of Mg concentration. No Mg was associated with the anion exchange resin indicating that the MgUO2(CO3)3(2-) complex did not sorb.