An ab initio direct molecular dynamics study of the reaction of CH3+ with benzene has been performed and the mechanism of reaction examined. Ab initio energy and gradient evaluations were done at the QCISD/6-31+G** and MP2/6-31+G** levels. The primary products of reaction were several isomeric forms of arenium ion (a complex). The reaction proceeds through a single channel, a direct barrierless insertion of a methyl cation into benzene to form a sigma complex. The direct insertion mechanism supports a recent quantum chemical study but differs from that posited in earlier experimental and theoretical studies of the reaction, which have assumed formation of a stable pi complex. The molecular dynamics simulation shows that product arenium ions are internally energetic enough for a hydrogen to shift about the benzenium ring to form several isomers. (C) 2004 Elsevier B.V. All rights reserved.