The ternary alkaline earth metal uranyl tricarbonate complexes, MnUO2(CO3)32n-4 (M = Mg and Ca), have been considered to be the major U(vi) species contributing to uranium mobility in natural water. Although MgUO2(CO3)32- can account for a substantial portion of U(vi) in a Mg2+-rich aqueous system and most processes regarding uranium are subjected to variable temperatures, chemical thermodynamic data for the prediction of the formation of MgUO2(CO3)32- at variable temperatures are still unknown. To fill the knowledge gap in the current chemical thermodynamic database, ultraviolet/visible (UV/Vis) absorption spectroscopy was employed to determine the formation constants (log K') of MgUO2(CO3)32- at varying temperatures of 10-85 °C in 0.5 mol kg-1 NaCl. The formation constants at infinite dilution, log K°, were obtained with specific ion interaction theory (SIT), and an increasing tendency of log K° with temperature was observed. Using calorimetric titration, the endothermic molar enthalpy of reaction (ΔrHm) of Mg2+ complexation with UO2(CO3)34- was determined at 25 °C. According to the chemical thermodynamic data obtained in this work, approximation models for the prediction of the temperature-dependent formation constant at a given temperature were examined and the constant enthalpy approximation with modification to the isoelectric reaction showed a satisfactory agreement with our experimental results. Finally, the effects of temperature on U(vi) speciation in Mg2+-rich groundwater and U(vi) extraction from seawater by amidoxime derivatives were examined. For the first time, this work provides important chemical thermodynamic data of MgUO2(CO3)32n-4 to assess the impact of temperature on U(vi) behaviour in groundwater and seawater.