A theoretical model based on the assumption about different rates of electron tunneling from different spin states is suggested to explain the experimentally observed low temperature inversion of OD ESR spectra of radical-ion pairs. Mathematical background of the model based on the conception of the effective tunneling spheres of different radii is given. The model suggested is in excellent agreement with the qualitative picture of OD ESR signal formation at the required conditions. On the basis of the model the inversion point of spectra is found and the behavior of the OD ESR signal intensity in the neighborhood of the inversion point depending on various physical parameters is predicted. The dependencies obtained may serve as a basis for experimental identification of the suggested mechanism.