The reactivity of chloromethane, bromomethane, and iodomethane over acidic zeolite catalysts has been probed, using the methylation of ethene, propene, and toluene as model reactions. Adsorbed reactants, transition states, and adsorbed products have been investigated. A cluster model has been used to represent the zeolite. Both the associative (direct) mechanism and the dissociative mechanism (via the formation of surface methoxide groups) have been studied. Quantum chemistry predicts that the associative pathway is favored over the dissociative mechanism. A weak trend in reactivity is found among the halomethanes, indicating the following order of reactivity: MeI > MeBr > MeCl. The results are compared with recent theoretical studies on methanol reactivity.