The major dissociation reactions of the cyclohexene radical cation, 1, lead to the cyclopentenyl ion by methyl loss and to ionized 1,3-butadiene after elimination of C2H4. These two reactions are also observed during the Diels-Alder reaction between ionized butadiene and ethylene in the gas phase. The energetic and mechanistic aspects of the methyl loss process from the cyclohexene radical cation or the reaction between ionized butadiene and ethylene are discussed with the help of molecular orbital calculations at the B3LYP/6-311+G(3df,2p)// B3LYP/6-31 G(d) levels. Methyl loss is demonstrated to result from successive 1,2-hydrogen shifts and ring-contraction/ring-opening steps involving, as a crucial intermediate, ionized bicyclo[1,3,0]hexane rather than the distonic ion [CH2CH2CHCHCHCH2].(+) (one of the open forms of ionized cyclohexene). This latter one is however involved during the direct and retro Diels-Alder reactions. The CH3 and C2H4 loss rate curves of the cyclohexene ion are calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM) equation and the molecular orbital calculation results. These estimations allow understanding of the experimental observations concerning dissociations of the cyclohexene radical cation, 1, and the collision complex formed between ionized butadiene and ethylene.