The X-ray diffraction study of the crystals deposited from the solution obtained by the reaction of magnesium with methanol allowed the formulation of the product as Mg(OMe) 2 3.5MeOH (I). Its structure is built of the residues of four types, namely cubane [Mg 4 (μ 3 -OR) 4 (OR) 4 (ROH) 8 ] neutral molecules, (Mg 4 (μ 3 -OR) 4 (OR) 2 (ROH) 1 0 ] 2 + cations, [(RO) 2 H] - anions (R = Me) and eight crystallographically independent non-coordinated solvating methanol molecules. All residues are linked into a three-dimensional framework by means of a complicated hydrogen-bonding system. The crystal structure provides the basis for discussion of the possible mechanism of desolvation of I.