The ongin of tbe hydrogen radical lost in the ionization chamber from the molecuiar ion of 2-methylindolizine has been studied by examination of the spectra of four specificay deuterated species. Hydrogen loss involves preferentially a hydrogen from the methyl substituent but aiso one of the hydrngens of eitber nng, espeaaüy those of the 5-membered ring. The HCN eliminotion from the metastable [M -H']' ions was studied using a linked scan method; the results are consistent with loss of identity of ali the hydrogen atoms of the precursor ion, whicb implies an extensive reorganization prior to fragmentation.