This study presents a theoretical exploration of the potential energy surface for the isomerization and dissociation to C7H7+ of ethylbenzene, and o-, m-, and p-xylene molecular ions. Quantum chemical calculations at the B3LYP/6-311+G(3df,2p)HB3LYP/6-31G(d) level indicated that isomerizations to methyl derivatives of the isotoluene ions could play important roles in the dissociations, particularly in the formation of the benzylium ion (Bz) from the xylene ions. It was predicted that Bz formation occurs mainly in a direct pathway from the ethylbenzene ion, while it does via the substituted isotoluene ions from the xylene ions. (c) 2006 Elsevier B.V. All rights reserved.