Abstract Tert-butyl-λ5-phosphanedione was generated by flash vacuum thermolysis (FVT) of trimethylsilyl tert-butyl phosphonohalidates. The structure of tert-butyl-λ5-phosphanedione was confirmed by chemical reactions. Tert-butyl-λ5-phosphandione readily undergoes trimerization, enters into the [2 + 3] - cycloaddition reaction with 2-phenyloxirane and adds alcohols.