The effect of spin- locking under the action of a resonance rf field of large amplitude has been observed in the optically detected ESR spectra (OD ESR) of radical-ion pairs both in strong and weak magnetic fields. "Hampering" of singlet-tgriplet transitions occurs for concentrations of aromatic acceptors in non polar solutions at room temperature from 10e-4 to 1 M. It is proposed that in spite of strong concentration narrowing OD ESR signals are inhomogeneously broadened. This can be due to neutral molecule- radical-anion aggregate formation.