Geminate recombination of radical ions of aromatic hydrocarbons in alkanes produces excited molecules: the relative yields of singlets and triplets change with time as the spin correlation decays. At short times ([graphic omitted] 50 ns) the process is a coherent one resulting from the isotropic hyperfine interaction with hydrogen nuclei: its rate is field-dependent and, for mobile liquids, it can be calculated exactly from e.s.r. data: deuteration will have a large effect and oscillations may be observed. At longer times, the yields became constant (but different) at high and zero field, but an additional small decay is predicted for very low fields. At still longer times, random spin relaxation will eventually make the yields independent of field. The theory is extended to cover initial production of triplet ion pairs in addition to singlets, the effect of charge transfer and recombination in viscous liquids and solids. It is applied to excited state production in radiolysis and to the radiation chemistry of pure alkanes.