ESR Spectroscopy has been used to study conformational interconversion and addivity relationships in a number of series of naphthalene derivatives. Oscillation of the alkyl rings in 1,2,3,6,7,8-hexahydropyrene ions, and the ions of the related 5, 6 and 7 membered ring compounds has been studied. The temperature dependance of the ESR spectra have been used to assign the coupling constants. The aromatic ring coupling constants in the asymmetric compounds have been assigned on the basis of an additivity relationship. The ESR spectra of a number of methylnaphthalene anions have been recorded and analysed to give the coupling constants. Improved additivity relationships for both the proton and methyl coupling constants have been determined. The temperature dependence of the spectrum of 1,2,3,4-tetramethylnaphthalene has been analysed. In terms of an interconversion between a tight and loose ion pair. The ESR spectra of a number of methylnaphthalene cations have been generated using AlCl₃/SO₂ and their spectra recorded and analysed. For those methylnaphthalenes which did not have methyl substituents in the 1 and 4 or 1 and 5 positions, a series of compounds with g=2.008 were recorded. Evidence is presented to show that these compounds are methyl derivatives of naphtho [1,8-cd]-1,2 dithiole. An additivity relationship for the proton coupling constants of these compounds has also been found. ESR spectra from the methylperylenes formed in the Scholl Condensation of the methylnaphthalene cations were also observed and analysed Changes in the ESR spectra of 1,8-dimethylnaphthalene and octamethylnaphthalene were ascribed to the effect of hindered rotation of the methyl groups. Coupling constants for the ESR spectra of tetracyclopentanaphthalene have been determined.