New radical cations of some methylated monocyclic aromatic hydrocarbons and their electron spin resonance spectra are reported. These spectra indicate that the degeneracy of the two highest bonding orbitals of the benzene system is removed by interaction with methyl groups. The parameters obtained from these cations, together with data from alkyl-substituted anthracenes and previously reported anions, indicate that the magnitude of the alkyl group interaction is an approximately linear function of the excess positive charge density in the cyclic system. This is taken to be evidence in support of the hyperconjugative mechanism as distinct from the spin polarisation mechanism for the interaction from alkyl groups in such systems.