Ion-molecule reactions and energy-resolved mass spectrometry have been used to determine the structures of the products formed in the reaction of diacetylene radical cation with ethylene in a flowing afterglow-triple quadrupole instrument. The structure of the adduct ion, C 6 H 6 . + , has been determined to be that of singly ionized benzene. The reaction thus presents a first example of the ability of diacetylene radical cation to undergo an aromatic ring forming reaction. The other products formed in the reaction are m/z 52, C 4 H 4 . + , and m/z 39, C 3 H 3 + . Isotopic labeling studies show that C 4 H 4 . + and C 3 H 3 + are formed with nearly statistical hydrogen incorporation, indicating a complex mechanism that scrambles all protons.