The mass spectra of several dibenzo (d.f)-1,3,2,-dioxaphosphorepines of the type where X=0, S, Se and R=2,2′-biphenylylene, have been studied. Dibenzochalcogen ions [RX]+˙ stemming directly from [M]+˙ were observed with high intensities, indicating specific migration of X from phosphorus to carbon with simultaneous destruction of the dioxaphosphorepine ring. Simple cleavage of P? Y as usually observed in similar organophosphorus compounds is not a common process here.