The addition reactions of tetramesitylgermasilene with 1-methoxybutadiene, ethyl vinyl ether, vinyl acetate, or styrene were studied. When tetramesitylgermasilene was allowed to react with 1-methoxybutadiene or styrene, formal [2+2] addition products were isolated. The addition of styrene to tetramesitylgermasilene was determined to be completely regioselective. In the presence of ethyl vinyl ether or vinyl acetate, tetramesitylgermasilene undergoes a 1,2-mesityl shift yielding a silylgermylene, at a faster rate than addition to either alkene. Tetramesityldisilene was also found to yield a formal [2+2] adduct with styrene. However, tetramesityldigermene rearranges to a germylgermylene at a faster rate than styrene addition.