The reactions of mass-selected [CH3NH21+' ions with the isomeric butenes and pentenes were studied at low collision energies in the radiofrequency-only quadruple collision cell of a hybrid B Q Q tandem mass spectrometer. Characteristic iminium ions arising by addition of the methylamine to the oiefin followed by fragmentation are observed for but-l-eae, pent-l-ene and Imethylbut-l-ene. However, for but-sene, pent-2-ene, 2-methylpropene, 2-methylbut-l-ene and %methylbut-2-ene, the major reaction channel of [ CH,NH2 I +' is charge exchange to form the olefinic molecular ion. The isomeric olefins are characterized, to a considerable extent, by the characteristic ion-molecule reactions that these molecular ions undergo with the neutral olefin.