The weak, polarized Raman band assigned to the v(1) CdO6 mode of the hexaaquo Cd (II) ion (O-h symmetry for the CdO6 unit, T-h with H-atoms) has been studied over the temperature range from 25 to 152 degrees C. The isotropic scattering geometry in R format was employed in order to measure the true vibrational contribution of the band and account for the Boltzmann temperature factor B and frequency factor. The band profile as a function of temperature has been examined analytically to extract the parameters : position of band maximum, full width at half-height (fwhh), integral intensity of the band, and relative molar scattering coefficient, S-h, over the temperature range measured. The dependence on concentration has also been measured. The 358 cm(-1) stretching mode of the hexaaquo Cd(II) shifts only 3 cm(-1) to lower frequencies but broadens about 32 cm(-1) for a 127 degrees C temperature increase. Two depolarized modes at 235 and 185 cm(-1) could be assigned to v(2)(e(g)) and v(5)(f(2g)), respectively. The Raman spectroscopic data suggest that the hexaaquo Cd(II) ion is thermodynamically stable in perchlorate solution over the temperature and concentration range measured, These findings are in contrast to CdSO4 solutions, recently measured by one of us, where sulfate replaces a water molecule of the first hydration sphere. Ab initio geometry optimizations of [Cd(OH2)(6)(2+)] were carried out at the Hartree-Fock and second-order Moller-Plesset levels of theory, using various basis sets up to 6-31+G*. The global minimum structure of the Cd(II) hexaaquo ion corresponds with symmetry T-h. The vibrational frequencies of the [Cd(OH)(6)(2+)]cation were also calculated. The unsealed frequencies of the CdO6 unit are lower than the experimental frequencies (ca. 16%), but scaling the calculated Hartree-Fock/MP2 vibrational frequencies (HF/6-31G*, HF/6-31+G*, and MP2/6-31G* levels) reproduces the measured frequencies. The theoretical binding energy for the hexaaquo Cd(II) ion was calculated and accounts for ca : 66% of the experimental hydration enthalpy of Cd(II). Frequency calculations are also reported for a Cd(OH2)(18)(2+) cluster with 6 water molecules in the inner sphere and 12 water molecules in the outer sphere. The global minimum of this cluster corresponds with symmetry T. The v(1) CdO6 mode (unscaled) occurs at 335 cm(-1), guile close to the experimental value. The water molecules of the first sphere form strong H-bonds with the water molecules in the second hydration shell because of the polarizing effect of the cadmium ion : The importance of the second hydration sphere is discussed.