The formation constants and specific ion interaction coefficients of MgUO2(CO3)32- complex were determined in 0.1 to 1.0 mol kgw-1 NaCl and 0.10 to 2.21 mol kgw-1 NaClO4 media in the framework of the specific ion interaction theory (SIT), by time-resolved laser-induced luminescence spectroscopy. The upper limits of ionic strength were chosen in order to limit luminescence quenching effects generated by high concentrations of Cl- and ClO4- already observed during our earlier studies on CanUO2(CO3)3(4-2n)- complexes (Shang & Reiller, Dalton Trans., 49, 466; Shang et al., Dalton Trans., 49, 15443). The cumulative formation constant determined is , and the specific ion interaction coefficients are ε(MgUO2(CO3)32-, Na+) = 0.19 ± 0.11 kgw mol-1 in NaClO4 and ε(MgUO2(CO3)32-, Na+) = 0.09 ± 0.16 kgw mol-1 in NaCl. Two gratings of 300 and 1800 lines per mm were used to acquire MgUO2(CO3)32- luminescence spectra, where the high-resolution 1800 lines per mm grating detected slight spectral shifts for the principal luminescent bands relative to CanUO2(CO3)3(4-2n)-. The applications of the consistent set of thermodynamic constants and ε values for MnUO2(CO3)3(4-2n)- (M = Mg and Ca) were examined in different geochemical contexts, where Mg over Ca concentration ratio varies to help defining the relative importance of these species.