The major dissociation reactions of the cyclohexene radical cation , 1, lead to cyclopentenyl ion by methyl loss and to ionized 1,3-butadiene after elimination of C 2 H 4. These two reactions are also observed during the Diels-Alder reaction between ionized butadiene and ethene in the gas phase. The energetic and mechanistic aspects of the methyl loss process from cyclohexene radical cation or reaction between ionized butadiene and ethene are discussed with the help of molecular orbital calculations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(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 [CH 2 CH 2 CHCHCHCH 2 ] •+ (one of the open forms of ionized cyclohexene). This latter is however involved during the direct and retro Diels-Alder reactions. The CH 3 and C 2 H 4 losses rate curves of the cyclohexene ion are calculated using the RRKM equation and the molecular orbital calculations results. These estimations allow the understanding of the experimental observations concerning dissociations of the cyclohexene radical cation , 1, and the collision complex formed between ionized butadiene and ethene. Ionized Cyclohexene 2