General conditions for the formation of heterometallic clusters by the simultaneous methanolysis of MoCl5 and MgCl2 were determined. The resultant alkalinity of the reaction solution, the Mg/Mo molar ratio, and the presence of traces of water are key factors responsible for the composition and structure of the mixed magnesium molybdenum methoxides that formed. The new decanuclear mixed-valence MoV,VI Mg oxomethoxide [MoV 4O4(μ3-O)2(μ2-O)2MoVI 2-O4(OMe)2Mg4(MeOH)6(μ3-OMe)6(μ2-OMe)8] (1) was synthesized by the reaction of lowernuclearity magnesium molybdenum oxoalkoxide complexes: NaMoV of the complex Na(MeOH)MoV 2O2(μ2-OMe)3(OMe)4 (2) and MgMoVI of the complex [MoVIO2Mg(MeOH)2-(OMe)4]2 (3). The molecular structure of 1 was determined by X-ray diffraction.