The fragmentation behaviour of organostannanes [SnPh,Et,-, (n = 0-4), SnPh,(CH,*CH,.Ph), spiro-[CH,], > Sn < [CH2I4, SnPh,X (X = F, CI, Br, or I), Sn2R, (R = Et, Ph)], organoplumbanes (PbEt,, PbPh4), and Ph4C have been examined under high resolution a t 70 ev. Parent ions are always of low abundance and decompose mainly by elimination of an odd-electron neutral fragment, whereas M R, + ions are of high abundance and lose even-electron fragments. Most of the ion current is carried by metal-containing species. Appearance-potential measurements on organostannanes have been used to evaluate djssociation-energy differences and dissociation energies in molecular ions.THE volatile alkyls of many elements were used by the mass spectra of a-bonded organometallic com-Aston in his classical isotope studies, but there have pounds. Dibeler and his co-workers 2-4 examined been comparatively few systematic investigations on several organomercury compounds, the tetramethyl derivatives of the Group IVB metals, and other lead V.