The germene Mes₂GeCR₂1 (Mes = 2,4,6-trimethylphenyl, CR₂ = fluorenylidene) reacts with 5-methoxy-1,4-naphthoquinone to yield, via the o-quinodimethane 8, the endoperoxyde 9 by simple reaction with molecular oxygen. By contrast, 1 with 2,3-dichloro-1,4-naphthoquinone gives the tetracyclic compound 7 by a double [2 + 4] cycloaddition between the GeC double bond and the conjugated system OC–CHCH. The new steric demanding germene Mes₂GeCR′₂5 (CR′₂ = 2,7-di-tert-butylfluorenylidene) undergoes similar [2 + 4] cycloadditions with various substituted or unsubstituted naphthoquinones, leading to tetracyclic adducts 10–12. The germene 5, the endoperoxide 9 and the cycloadducts 10 and 12 have been structurally characterized.