The thermolysis and photolysis of hexamesitylcyclotrigermane in the presence of ethylmagnesium bromide has been investigated. Under photochemical conditions, ethyldimesitylgermane, 1,2-diethyl-1,1,2-trimesityldigermane and ethyl-1,1,2,2-tetramesityldigermane were isolated and, under thermal conditions, 1,2,2-triethyl-1,1-dimesityl digermane and 2,2-diethyl-1,1,1-trimesityldigermane were isolated. The photolysis of hexamesitylcyclotrigermane in the presence of methyllithium has also been investigated. In both cases, the organometallic reagent adds to tetramesityl digermene and dimesitylgermylene formed by photochemical or thermal cleavage of the cyclotrigermane. In the case of the addition of the Grignard reagent, the resulting germyl Grignard reagent undergoes a facile ligand exchange reaction.Key words: digermene, germylene, Grignard reagents, alkyllithium reagents, germylmagnesium compounds, germyllithium compounds.