Infrared photodissociation spectra of [Mg.(H2O)(1-4)](+) and [Mg.(H2O)(1-4).Ar]+ are measured in the 30003800 cm(-1) region. For [Mg.(H2O)(1-4)](+), cluster geometries are optimized and vibrational frequencies are evaluated by density functional theory calculation. We determine cluster structures of [Mg.(H2O)(1-4)](+) by comparison of the infrared photodissociation spectra with infrared spectra calculated for optimized structures of [Mg.(H2O)(1-4)](+). In the [Mg.(H2O)(1-3)](+) ions, all the water molecules are directly bonded to the Mg+ ion. The infrared photodissociation spectra of [Mg.(H2O)(4)](+) and [Mg.(H2O)(4).Ar](+) show bands due to hydrogen-bonded OH stretching vibrations in the 3000-3450 cm(-1) region. In the [Mg.(H2O)(4)](+) ion, three water molecules are attached to the Mg+ ion, forming the first solvation shell; the fourth molecule is bonded to the first solvation shell. As a result, the most stable isomer of [Mg.(H2O)(4)](+) has a six-membered ring composed of the Mg+ ion, two of the three water molecules in the first solvation shell, and a termination water molecule.