The ternary alkaline earth metal uranyl tricarbonate complexes, MnUO2(CO3)(3)(2n-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)(3)(2-) 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)(3)(2-) 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)(3)(2-) at varying temperatures of 10-85 degrees C in 0.5 mol kg(-1) NaCl. The formation constants at infinite dilution, log K degrees, were obtained with specific ion interaction theory (SIT), and an increasing tendency of log K degrees with temperature was observed. Using calorimetric titration, the endothermic molar enthalpy of reaction (Delta H-r(m)) of Mg2+ complexation with UO2(CO3)(3)(4-) was determined at 25 degrees 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)(3)(2n-4) to assess the impact of temperature on U(vi) behaviour in groundwater and seawater.