Photoirradiation of polysilacycloalkynes in the presence of acetone provides the acetone adducts of bicyclic silacyclopropenes. Ringcontraction of polysilacycloalkynes proceeds via two different pathways; one is a process via bicyclic silacyclopropene formed by rearrangement of the silicon atom adjacent to acetylene carbons and another is a direct desilylation process.