Various mechanisms of geminate recombination in spin-correlated radical ion pairs produced by radiolysis of organic solids under cryogenic conditions are considered and compared with experimental data. Single-step electron tunneling and slow resonant hole transfer are examined from the viewpoint of their manifestation in pulsed time-resolved fluorescence-detected magnetic resonance (FDMR). Two theoretical models treating the phenomenon are developed, and their predictions are compared with the experimental results. These models are found to be consistent with many observed features : t(-1) decay kinetics, unusual evolution of the FDMR spectra with time, narrow radial distribution of the FDMR-active pairs, and efficient spin memory transfer from the primary pairs. However, with very few exceptions the tunneling theories fail to reproduce the exact scale of the effect and its dependence on solute concentration and thermalization distance of electrons. This discrepancy suggests that the negative charge is more mobile than predicted by the single-step tunneling model.