Multiquantum resonances in low-field optically-detected electron spin resonance (ESR) spectra of radiation-induced radical-ion pairs of aromatic acceptors in liquid and some polymeric solutions have been studied experimentally and theoretically. Corresponding resonances occur in spectra under the action of relatively intense radiofrequency (RF) pumping, when usual signals are inverse due to spin-locking effect. Special experiments have been carried out to prove that such resonances are not caused by the apparatus. The dependencies of resonance shifts and intensities on the radiofrequency field amplitude for various polarizations of the RF field have been studied. The case of pumping with two independent RF generators has also been analysed. A general theory for multiquantum optically detected ESR spectra of radical-ion pairs in liquid solutions has been developed, with spin relaxation processes being neglected. The theory has been applied to calculating multiquantum resonance parameters in transverse RF fields (H1⊥Ho) of various polarizations. The mechanism of multiquantum resonance spin dynamics of radical pairs (RP) has been analysed in detail. The shapes of OD ESR spectra components have been studied for the Poisson distribution of RF lifetimes (47 refs.)