Radiolysis of liquid hydrocarbons produces radical cations and electrons. Their recombination releases sufficient energy in the solvent, or in the solute if a charge scavenger is added, to produce excited states, Because of the low dielectric constant, most of the recombination isgeminate, i.e. the positive and negative charges do not separate as far as the Onsager escape distance (1∼30nm for an alkane at room temperature). For a single ion pair, this means that the two unpaired electrons were initially paired in a molecular singlet state. If recombination is very fast (< 1-10ns), the spin correlation is retained: only singlet products will result. Over longer periods, the correlation decays because of hyperfine interaction between the electrons and magnetic nuclei. This leads to time-dependent magnetic field and magnetic isotope effects on the product yields, easily detected by studying the fluorescence in suitable systems. Spin relaxation randomizes spin orientations but the hyperfine effect is a coherent process and oscillations can be detected. Theory and experiment are in good agreement in most respects. Electron spins can be reversed by absorption of microwaves: the changes in fluorescence can be used for spectroscopic studies of the ions.