Records of methanesulfonate acid in ice cores play an important role in reconstruction or the past history of marine productivity, sea ice extent, and major El Nino event activities related to climate changes. Considering the lack or thermodynamic and crystal structure data, crystallization conditions below zero degrees Celsius were mimicked in the laboratory, and magnesium methanesulfonic hydrate crystals, Mg(CH3SO3)(2)center dot 12H(2)O (i.e., [Mg(H2O)(6)](CH3SO3)2 center dot 6H(2)O) were grown from solution by cooling crystallization and by eutectic freeze crystallization. The solubility lines between -5 and +21 degrees C and between 0 and 25 wt % are presented. The eutectic point of the system is detected at -5 degrees C and 14 wt %. The crystal structure analysis and the molecular arrangement or these crystals were determined using single crystal X-ray diffraction (XRD). Reflections were measured at a temperature of 110(2) K. The structure is trigonal with.space group R (3) over bar (no. 148). The crystal is it colorless Hoek with the following parameters: a = b = 9. 27150(8) angstrom. c = 21.1298(4) angstrom. V = 1572.99(4) angstrom. Z = 3, and D-calc = 1.364 g/cm(3). The Raman spectrum of Mg(CH3SO3)(2)center dot 12H(2)O salt has been recorded and the principal absorption modes identified. Thermogravimetric analysis confirmed the stochiornetry of the Mg(CH3SO3)(2)center dot 12H(2)O salt.