Two formulae have been derived relating the E.S.R. hyperfine coupling constant of methyl and methylene group protons in aromatic radicals to the pi-electronic structure of the radical. The derivation of these formulae assumes that the principal coupling mechanism is hyperconjugation but includes a small spin polarization term. The formulae have been used to calculate theoretical methyl and methylene coupling constants in several radicals whose E.S.R. spectra have been measured.