Abstract Treatment of 4-t-butyl-2, 6, 7-trioxa-1-phosphabicyclo [2, 2, 2] octane with sulphuryl chloride yields merely its P-oxide and not the expected 5-t-butyl-2-chloro-5-chloromethyl-1, 3, 2-dioxaphosphorinan 2-oxide. The mass spectra of the bicyclic phosphite and its oxide are discussed.