In the present article, the hydration of , ion-associated species and process in the Mg(ClO₄)₂ solution were investigated at the HF/6-31＋G* level. The first aqueous sphere can be filled by 6 water molecules at least. When the hydrated number is less than 6, the blue-shift of the v₁- frequency of 1.7 cm^－1 can be resulted by each added water molecule. The direact contact of the first Mg^2＋ can lead the v₁- frequency a large red-shift, and the second and the third Mg^2＋ will make the reversed shift. The shifts will be contributed to the polarization of the Mg^2＋. Some associated species will form between and aqueous Mg^2＋, such as solvent-shared ion pairings, contact ion pairings and triple ion clusters. In these species, the solvent-shared ion pairing and triple ion cluster with one aqueous Mg^2＋ and two ions can lead the v₁- frequencies to small blue-shifts which are corresponding with the Raman spectra in experiment. The ion associated process which is not analogous with the MgSO₄ but Mg(NO₃)₂ system, is predicted to be: free ions→solvent-shared ion pairing→solvent-shared triple ion cluster with one aqueous Mg^2＋ and two ions→Mg(ClO₄)₂•6H₂O (crystal).

Key words: quantum chemical calculation, magnesium perchlorate, ion-associated species, associated process