The weak polarized Raman band assigned to the nu(1)-MgO6 mode of the hexaaquo Mg(II) ion has been studied over the temperature range 25 to 125 degrees C. The 356 cm(-1) stretching mode frequency decreases by about 3 cm(-1) but broadens by 13 cm(-1) over a 100 degrees C temperature range. A depolarized mode at 235 cm(-1) could be assigned to nu(2). These data suggest that the hexaaquo Mg(II) ion is thermodynamically stable in perchlorate and chloride solutions. In sulfate solutions, an equilibrium exists between the hexaaquo ion and an innersphere sulfate complex. Ab initio geometry optimizations of Mg(H2O)(6)(2+) were carried out at the Hartree-Fock and Moller-Plesset levels of theory, using various basis sets up to 6-31+G*. Frequency calculations confirm that the T-h structure is a minimum. The unscaled frequencies of the MgO6 unit are lower than the experimental frequencies, and scaling only marginally improves the agreement. The theoretical binding enthalpy for the hexaaquo Mg(II) ion accounts for about 70% of the experimental hydration energy of Mg(II). A comparison of three models for the second hydration sphere is presented, and the most suitable is found to be one of lower symmetry T, in which alternate faces of the MgO6 octahedron are H-bonded to water trimers. The unsealed Hartree-Fock frequencies agree very well with our experimental observations, giving nearly exact agreement with experiment.