Reagent gas systems are described for use in a single-source chemical ionization mass spectrometer which permit charge exchange mass spectra to be obtained using CS2+•(∼10.2 eV), COS+•(11.2 eV), Xe+•(∼12.5 eV), CO+•(14.0 eV), N2+•(15.3 eV), and Ar+• (15.8 eV) (recombination energies in brackets) as the major reactant ions. Using these reagent gas systems the charge exchange mass spectra of cyclopentane, 1-pentene, 3-methyl-l-butene, 2-methyl-l-butene, 2-pentene, and 2-methyl-2-butene have been obtained. It is shown that these spectra can be used to construct breakdown graphs which adequately depict the relative energy dependences of the fragmentation pathways and rationalize the 70 eV electron impact mass spectra.