The Mg(II) complexes of the nitro-substituted benzoic acids have been investigated by thermal and spectroscopic methods. Thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), and other spectroscopic methods have been used to investigate the thermal behavior and structure of the compounds [Mg(H2O)(6)](4-nba)(2)(.)2H(2)O (1) under bar, [Mg(H2O)(6)](3-nba)(2)(.)2H(2)O (2) under bar, [Mg(H2O)(6)](2-nba)(2)(.)2H(2)O (3) under bar (4-nba = para-nitrobenzoate; 3-nba = meta-nitrobenzoate; and 2-nba = ortho-nitrobenzoate). All the complexes are formulated as consisting of-the octahedral [Mg(H2O)(6)](2+) cation with the carboxylates outside the coordination sphere, which is consistent with the IR, UV-Vis and H-1 NMR data. Thermal decompositions of these carboxylates are multi-stage processes. The composition of the complexes and the solid-state intermediates and the resultant products of thermolysis have been identified by IR, elemental analysis and complexometric titration. A possible scheme for the decomposition of the complexes is suggested. Heating the compounds first results in the loss of water molecules. This is followed by the loss of the nitrobenzoate resulting in the formation of oxide. Complex (1) under bar can be reversibly hydrated. The final product of the thermal decomposition in all the cases is MgO.