The formation and behaviour of ion pairs have been examined by esr spectroscopy for the pyrene, perylene, naphthalene and dimethylnaphthalene systems. The alkali metal hyperfine splittings and the linewidths, in particular, provide much information on the nature and dynamics of these ion pairs. For some systems thermodynamic and kinetic data have been obtained. Various Molecular Orbital calculations were carried out and from these the most likely position of the alkali metal ion in the ion pairs is suggested. The trends in ion pair formation with cation, anion, solvent and temperature are discussed. In the symmetric dimethylnaphthalene series, the effect of the methyl groups on the counter-ion position is considered, with the 1,4- and 2,3- systems being particularly interesting. The hyperfine splittings of the unsymmetric dimethylnaphthalenes allow an additivity model for methyl-group substitution to be completed. Five of the six ¹³C coupling constants for the pyrene anion were determined.