The photolysis of 1 produces the carbene 2, which is transformed into the short lived phosphene 4 by phenyl migration. With the α, β-unsaturated ketones 6a–d in a [2+2] cycloaddition 4 yields the 1,2λ5-oxaphos-phetanes 7a–d The heterocycles partially undergo a photofragmentation, which produces the olefines 9a–d and the heterocumulene 8. If 4 is generated from 1 under thermal conditions in the presence