Methylmagnesium iodide (or bromide) has been found to add to tetramesitylgermasilene and tetramesityldigermene. The addition to the germasilene is regioselective, with the methyl group adding to the silicon end of the silicon-germanium double bond to produce a germylmagnesium iodide (or bromide) species. The germyl Grignard reagents give the corresponding germanes upon hydrolysis. At 110 degrees C in the presence of excess methylmagnesium halide, the mesityl substituents on the germyl Grignard reagent germanium atom were exchanged for methyl groups. A mechanism involving the a-elimination of MesMgX followed by the addition of MeMgX to the intermediate germylene has been proposed to explain the observed ligand exchange. Evidence for the presence of intermediate germylenes has been obtained in trapping experiments with 2,3-dimethylbutadiene.