The structure of the addition product formed in the reaction of the trans-1,3-butadiene radical cation with ethylene is examined by using a flowing afterglow -triple quadrupole instrument. Energy-resolved collision-induced dissociation studies show the product not to be the cyclohexene radical cation but likely the 2,4-hexadiene radical cation. It is shown that thermodynamics accounts for the preference of the 2,4-hexadiene ion over the cyclohexene ion but does not account for the preference of the 2,4-hexadiene ion over the 1-methylcyclopentene ion. Reaction of the 1-bromobutadiene radical cation with ethylene suggests but does not require that the cyclohexene cation be accessed as an intermediate in the reaction. It is suggested that the mechanism involves the concerted addition of ethylene to a C-H bond in the butadiene cation, as has been described previously for the cis ion.