Crystals suitable for X-ray structure analysis were obtained after the slow evaporation of the reaction mixture containing equimolar quantities of magnesium chloride and dodecatungstophosphoric acid aqueous solution insuring pH of the solution between 1.0 and 1.2. This simple synthetic route yielded stability of Keggin anion and high quality [Mg(H2O)6][HPW12O40]·4H2O single crystals. The obtained compound belongs to the group of heteropoly compounds and its structure is composed of Keggin [PW12O40]3– anions, [Mg(H2O)6]2+ cations and lattice water molecules. Zigzag arrangement of Keggin anions along c-axis creates irregular channels occupied by [Mg(H2O)6]2+ cations and lattice H2O molecules. The calculation of the total potential solvent volume indicated the presence of 4.1 lattice H2O lattice molecules per formula unit, which is in agreement with the here presented structural model. The position of one lattice water molecule is well defined, while each of three other molecules is statistically distributed over two locations. Hydrogen bonds involve all coordinated and lattice H2O molecules, as well as some oxygen atoms from the Keggin anion.