The mechanism of the addition of aldehydes to disilenes and germasilenes was investigated by examining the reaction between trans-(2-phenylcyclopropyl)carboxaldehyde and tetramesityldisilene or tetramesitylgermasilene. [Dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl] dimesitylsilane (1) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-disilacyclohept-6-ene (2) were formed in the reaction between the aldehyde and the disilene, and the analogous [dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl] dimesitylgermane (4) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-silagermacyclohept-6-ene (5) were formed in reaction with the germasilene. It is proposed that compounds 1, 2, 4, and 5 are the result of either disproportionation or ring closure of an intermediate biradical derived from initial bond formation between the oxygen atom of the aldehyde and the doubly bonded silicon atom of the dimetallene.