The energetics and dynamics of the protonation of the naphthalene molecule were investigated at the Hartree-Fock (HF) and correlated levels. Calculations at correlated levels reproduced the experimental gas-phase proton affinity and basicity of naphthalene quite well. Positional isomerizations of monoalkylnaphthalene were studied at various theoretical levels. The calculated; thermodynamic equilibrium compositions at correlated levels are in good agreement with the experimental results. According to the calculations, intramolecular shifts are possible for the methyl, ethyl, and isopropyl groups. By contrast, the bulky tert-butyl group transfers exclusively intermolecularly. In accordance with the experimental results, the activation energies of the 1,2-alkyl shifts decrease with the increase in the size of the alkyl group. This indicates that the rate-determining step of the isomerization reactions is the rearrangement of the monoalkylnaphthalenium ions via 1,2-alkyl shifts. An intermolecular methyl group transfer between two naphthalene molecules is energetically much less favored than the relevant intramolecular one. However, an appropriate basis set superposition error correction to the total energy of the three-body transition state complex is of utmost importance at second order M phi ller-Plesset level.