The alkylation of m-xylene by Me(3)C(+) ions has been studied in the gas phase with the aim of assessing the kinetic role of proton transfer from intermediate arenium ions. Hydrogen kinetic isotope effects (KIEs) emerging from the reaction of m-xylene-4-d and of m-xylene/m-xylene-d(10) mixtures have been exploited as mechanistic probes. The Me(3)C substitution at the 4-position displays a base-strength-dependent KIE related to rate-determining deprotonation of arenium intermediates. The Me(3)C(+) reaction at the 5-position is instead characterized by a base-independent KIE due to 1,2-hydrogen shift to form a highly stable isomeric arenium ion. When the latter species originates from m-xylene-4-d, its neutralization involves an intramolecular competition of proton vs deuteron abstraction, manifesting a net primary KIE. Factors affecting its magnitude and base-strength dependence are discussed.