The Letter presents a method of describing the process of single charge transfer in the partner of a radical ion pair, accompanied by changes in its hyperfine structure. In the particular case of 'freezing' of spin evolution in the pair due to collapse of hyperfine interactions upon single charge transfer and in the approximation of exponential recombination kinetics the suggested model describes MARY spectra for three important classes of processes in nonpolar solutions: chemical decay of the pair partner, 'dark capture' of the partner, and slowing down of spin evolution in the partner upon abrupt narrowing of its ESR spectrum. (C) 2003 Elsevier B.V. All rights reserved.